Is Pus Impure?
Question

Is a spot of pus that is yellow or white impure, whether it is solid or liquid?
Praise be to Allah.“Pus is a viscous yellow liquid that comes from wounds and the like because of some corruption or infection." (Mu`jam Lught Al-Jurists p. 373).
Serous discharge is a thin liquid that may be mixed with blood, before it thickens and turns into pus. (See: Tilbat At-Talabah, p. 22; Al-Mawsu`ah Al-Fiqhiyyah, 21/25)
Serous discharge may appear in the wound before pus.
The ruling on pus and serous discharge is the same as the ruling on blood , according to the majority of the jurists of the four schools and others (may Allah have mercy on them), with regard to impurity and overlooking a small amount, because pus and serous discharge are originally blood, that have turned into something rotten and corrupt. Therefore if blood is impure, then pus is more likely to be impure. (See: Bada’i‘ As-Sana’i‘ (1/60); Al-Majmu`, 2/558; Al-Qawanin Al-Fiqhiyyah, p. 27)
Pus comes from blood , and what stems from something comes under the same ruling as its origin.
We have previously discussed the impurity of blood in the answer to question no. 114018 .
It says in Al-Mawsu`ah Al-Fiqhiyyah (34/128): 
“The jurists agreed that if pus is expelled by the human body, it is impure, because it comes under the heading of unclean things (Al-khaba’ith: unclean, impure, evil things). Allah, may He be Exalted, says (interpretation of the meaning): {… and prohibits for them bad things…} [Al-A‘raf 7:157] And people of sound nature regard it as dirty. The prohibition in this case is indicative of impurity, because there is a reason to regard pus as impure, as impure is a word that refers to what is regarded as repulsive, and pus is something that people of sound nature find repulsive, because it becomes dirty and foul-smelling, and because it comes from blood, and blood is impure.”
Ibn Qudamah Al-Maqdisi (may Allah have mercy on him) said: pus, serous discharge and whatever originally comes from blood is to be regarded as being like blood, except that Ahmad said: It is not as serious as blood.
It was narrated from Ibn ‘Umar (may Allah be pleased with him) and Al-Hasan (may Allah have mercy on him) that they did not think that it was like blood.
Abu Mijlaz (may Allah have mercy on him) said regarding serous discharge: Allah only mentioned blood poured forth."(Al-Mughni,  2/483).
And he (may Allah have mercy on him) said: “Based on that, a greater quantity of it may be overlooked than in the case of blood, because its quantity is not regarded as being too great unless it is greater than the quantity that may be overlooked in the case of blood, and because there is no religious text concerning it. Rather it is regarded as impure because it is transformed from blood and has become a repulsive substance." (Al-Mughni by Ibn Qudamah, 2/484)
Imam Ahmad (may Allah have mercy on him) was asked: Are blood and pus the same in your view? He said: No; the scholars did not differ concerning blood, but they differed concerning pus. On one occasion he said: Pus and serous discharge are not as serious as a blood." (Ighathat Al-Lahfan, 1/151)
Shaykh Al-Islam Ibn Taymiyah (may Allah have mercy on him) favoured the view that pus and serous discharge are pure, as he said: “It is not required to wash the garment and the body from pus and serous discharge, and there is no proof that they are impure." (Al-Ikhtiyarat Al-Fiqhiyyah,  p. 26).
Undoubtedly the view of the majority of scholars is more prudent and better to absolve oneself of blame, except in the case of a small amount which may be overlooked, especially when it is difficult to protect oneself from it and it becomes a problem, as usually happens in the case of those who are sick or injured. What appears to be the case in the situation asked about, which refers to a spot, is that it is a small amount that has not reached the extent that is to be regarded as being too much.
In Fatawa Al-Lajnah Ad-Da’imah it says: “Blood , pus and serous discharge may be overlooked in small amounts, provided that they come out from anywhere other than the private part, because avoiding such a small amount is too difficult." (Fataawa Al-Lajnah ad-Daa’imah  5/363)
And Allah knows best.