The Hadith About Allah Coming in a Form Muslims Do Not Recognize
Question

What is the meaning of this part of a lengthy hadith
Will Allah take a different form rather than his original form?
Did Allah descended to earth to talk to Musa or did he spoke from above the throne? Did Allah took form of fire when he talked to him
Is taking form of a human being attribute of Allah?
Praise be to Allah.Firstly:
The Sunnah indicates that people will see their Lord on the Day of Resurrection during the reckoning, and the Prophet (peace and blessings be upon him) will see Him when he intercedes for creation at the time of judgment. This is the first vision.
Then, when the disbelievers are taken to the Fire, and this Ummah remains, including its hypocrites, Allah will come to them in a form different from the one they saw Him in the first time, and this is a test and trial for them. He will ask them what they are waiting for, and they will say they are waiting for their Lord. He will ask if there is a sign between them and Him, and they will say: the Shin. Then He will reveal His Shin, and the believers will prostrate. When they raise their heads, they will see Him in His first form.
As for the first vision: it is evidenced by the Hadith of `Adi ibn Hatim reported by Al-Bukhari (1413) which includes the Prophet's statement (peace and blessings be upon him): "Then each one of you will stand before Allah without a veil or an interpreter to translate for him," and the Hadith of Abu Hurairah reported by Muslim (2968): "Then the servant will meet [Allah], and He will say: 'O so-and-so, did I not honor you, and make you a leader, and marry you, and subjugate horses and camels for you, and allow you to be in charge and to have comfort?' He will say: 'Yes.' [Allah] will say: 'Did you think that you would meet Me?' He will say: 'No.' [Allah] will say: 'Then I will forget you as you forgot Me.'" The meeting includes seeing, then the veiling occurs for the disbelievers. See: Majmu` Al-Fatawa by Shaykh Al-Islam Ibn Taymiyah (6/462-468).
And in the Hadith of Anas regarding intercession reported by Al-Bukhari (7440) and Muslim (193): "Then I will ask permission to approach my Lord, and permission will be granted to me. When I see Him, I will fall prostrate..." This confirms the Prophet's (peace and blessings be upon him) vision of his Lord on the Day of Resurrection before Allah comes to the believers in a different form, and it reinforces the affirmation of the first vision.
It has been reported by Ahmad (15), Ibn Khuzaymah in At-Tawhid (2/735), and Ibn Hibban (6476) from the Hadith of Hudhayfah about Abu Bakr: "...go to Muhammad (peace and blessings be upon him), so he may intercede for you with your Lord, the Mighty and Majestic. He said: 'Then he will go, and Jibril (peace be upon him) will come to his Lord, and Allah, the Mighty and Majestic, will say: 'Grant him permission, and give him the glad tidings of Paradise.' He said: 'Then Jibril will take him, and he will fall prostrate for the duration of a Friday, and Allah, the Mighty and Majestic, will say: 'Raise your head, O Muhammad, speak and you will be heard, and intercede and your intercession will be accepted.' He said: 'Then he will raise his head, and when he looks at his Lord, the Mighty and Majestic, he will fall prostrate for the duration of another Friday, and Allah, the Mighty and Majestic, will say: 'Raise your head, speak and you will be heard, and intercede and your intercession will be accepted.'" And its chain of narration is sound as stated by Sheikh Shu`aib Al-Arnaut in his verification of Ibn Hibban.
And Ibn Hibban (6480) reported from the Hadith of Anas: "Then his Lord will manifest to him, and no prophet before him will have seen Him. He will fall prostrate to Allah, and praise Him with praises that no one before him has praised Him with, and no one after him will praise Him with." And its chain of narration is sound.
As for the second and third visions: in the Hadith of Abu Sa`id reported by Muslim (183): "...the Lord of the Worlds, Glorified and Exalted, will come to them in a form not recognisable to them. He will say: 'What are you waiting for? Every nation will follow what they used to worship.' They will say: 'Our Lord, we parted with people in the world when we were most in need of them, and we did not accompany them.' He will say: 'I am your Lord.' They will say: 'We seek refuge with Allah from you; we do not associate anything with Allah.' This will happen two or three times, until some of them are almost turned away. He will say: 'Is there a sign between you and Him by which you will recognize Him?' They will say: 'Yes.' Then the Shin will be revealed, and no one who used to prostrate to Allah of his own accord will remain except that Allah will permit him to prostrate, and no one who used to prostrate out of fear and showing off will remain except that Allah will make his back a single plate, every time he tries to prostrate, he will fall on his back. Then they will raise their heads, and He will have changed into the form they saw Him in the first time. He will say: 'I am your Lord.' They will say: 'You are our Lord.' Then the bridge will be set over Hell."
And in the Hadith of Abu Sa`id reported by Al-Bukhari: "He said: 'Then the Compeller will come to them in a form other than the one they saw Him in the first time. He will say: 'I am your Lord.' They will say: 'You are our Lord.' And no one will speak to Him except the prophets. He will say: 'Is there a sign between you and Him that you recognize?' They will say: 'The Shin.' Then He will reveal His Shin, and every believer will prostrate to Him, and those who used to prostrate to Allah out of showing off and seeking reputation will remain, and when they go to prostrate, their backs will become one plate." Meaning their backbones will become even, and they will not be able to bow for prostration.
The form in which Allah comes to His servants is unknown to us; it is part of the unseen that He has not informed us about. We believe in it and submit, and we do not indulge in our whims, nor do we imagine, nor do we liken, nor do we represent, for He, Glorified be He, "cannot be grasped by the minds through contemplation, nor can the hearts imagine Him through picturing, {There is nothing like unto Him, and He is the All-Hearing, the All-Seeing} [Ash-Shura: 11]." End quote from Lum`at Al-I`tiqad by Ibn Qudamah, p. 5.
Secondly:
Ibn Al-Qayyim (may Allah have mercy on him) affirmed that this light is the light of the veil, saying: "The light with which He veiled Himself is called light and fire, as there is hesitation in its expression in the authentic Hadith, the Hadith of Abu Musa Al-Ash`ari, which is his statement: 'His veil is light or fire.' Indeed, this fire is light, and it is the one in which Allah spoke to His messenger Moses, and it is a pure fire that has brightness without burning." End quote from Mukhtasar As-Sawa`iq, p. 423.
And the fact that the light in the tree is the light of Allah the Almighty does not contradict His highness above His Throne, but it indicates closeness to what is below the heavens.
Shaykh Al-Islam (may Allah have mercy on him) said: "And if the caller is Allah, Lord of the Worlds, and He called him from a specific place and brought him close; this indicates what the Salaf said: of His closeness and nearness to Musa (peace be upon him); even though this closeness is from what is below the heavens." End quote from Majmu` Al-Fatawa (5/464).
And what is well-known about the descent among the Salaf is that they say: He descends without the Throne being empty of Him, and this indicates that His descent is not like the descent of the created beings, Glorified and Exalted be He, and Sanctified. He descends to the lowest heaven and there is nothing above Him, and no one knows how He descends except Him, Glorified be He.
And Shaykh Al-Islam (may Allah have mercy on him) said: "And the basis of this: that His closeness - Glorified be He - and His nearness to some of His creatures, does not necessitate that His essence is empty from above the Throne. Rather, He is above the Throne, and He draws near to His creation as He wills, as those who said it from the Salaf said, and this is like His closeness to Musa when He spoke to him from the tree." End quote from Sharh Hadith An-Nuzul, p. 99.
And it does not entail any problematic issue such as something encompassing Him, but He, the Exalted, is high in His nearness, above everything.
For this reason, Ibn Kathir (may Allah have mercy on him) said in this context: "And His statement (translation of the meaning): {And Glorified be Allah, Lord of the Worlds} means:  He who does what He wills and does not resemble any of His creatures, and nothing of His creations encompasses Him, and He is the Most High, the Great, distinct from all creatures, and the earth and the heavens do not contain Him, but He is the One, the Eternal Refuge, sanctified from resembling the created things." End quote from Tafsir Ibn Kathir (6/180).
Thirdly:
Musa (peace be upon him) did not see his Lord, and Allah is exalted above taking the form of fire or a human, for Allah, Glorified be He, is unlike anything.
And Allah knows best.