Forgetting, for the moment, looking for a translation for 'password' and instead looking at how the sentence 'all passwords must die' works:
The phrase 'udrir morghulis' ('udir' means 'word' and so 'udrir' means 'all words') doesn't technically translate to saying 'all words must die' but actually states the 'fact' that 'all words die' - 'valar morghulis' is actually the fact that 'all men die' (interesting and irrelevant aside: this explicitly excludes women - the phrase 'all people die' is actually 'abrar morghulis', which is also 'all women die'). Being extremely pedantic, it's really stating the fact that 'all men are currently going towards death'
So, there are a few ways to go about this:
1. Create a valyrian word for password and put this in the statement that 'all passwords are going towards death'
2. Create a valyrian word for password and put this in the command that 'all passwords must die' (easy once 1. has been done)
3. 242
Create a statement that says something along the lines of 'all words that allow passage die'
4. Create a command that says something along the lines of 'all words that allow passage must die'
To create a recognisable meme or somesuch, 1. would be the best option.
First of all, creating something to mean password: the verb 'to pass' is 'rēbagon' and the noun 'word' is 'udir', so the simplest hack is just to use 'rēb' (the stem) and add 'udir' to the end. So The Valyrian word for 'password' is (probably) 'rēbudir' Proncounced [reː'budir] and is a 5th declension, aquatic noun.
From this, 'All passwords are going towards death' translates to 'rēbudrir morghulis'
As 'morghūljagon' is a consonant-final verb, commanding all passwords to die is given by 'rēbudrirzi morghūljās'
Now, forgetting about the word for 'password', translating the phrase 'all words that allow passage':
* As before, 'all words' = 'udrir'
* 'to allow' = 'gaomagon' (consonant-final, stem = 'gaom')
* 'to pass' = 'rēbagon' (consonant-final, stem = 'rēb')
Turning this into a relative clause (see comments) gives 'rēbagon gaomis luor udrir' as 'all words which allow to pass'.
So, the phrase 'all words that allow to pass are going to death' is 'rēbagon gaomis luor udrir morghūlis' and giving permission for 'all words that allow to pass may die' is 'rēbagon gaomis luor udrir morghūlis kostas'
Finally, turning this into a command gives 'all words that allow to pass must die [go to death]' as 'rēbagon gaomis luos udrirzi morghūljās'
All of the information used above has been taken from the Tongues of Ice and Fire Wiki
For completeness (unnecessary as it was pre-edit, but potentially interesting):
The word 'passage' (or anything that's equivalent) is the noun of the verb 'to pass', which is 'rēbagon' as before. To turn this into a noun, just add 'non' to the stem:
* 'passage' = 'rēbnon' (3rd declension, terrestrial)
Putting 'word' as the subject and 'passage' as the object creates the phrase 'udrir rēbnon gaomisi' as 'words allowing passage'. 'current words allowing passage' would be 'udrir rēbnon gaomis' 312