I understand your reluctance. You're afraid of damaging the eye in case there's an eye left. This is why I suggested manipulating the part at the outer edge away from the center where the vision is. Accidentally scratching the sclera won't affect vision. But I believe there is so much pus, you won't come anywhere near the actual surface of the eye in just trying to lift off the outer skin of the pus.
Take this into consideration. If you were a lone hiker and a boulder fell on your arm trapping you, could you cut off your arm to save yourself? True story.
https://en.wikipedia.org/wiki/Aron_Ralston Sometimes we have to take a big risk to avoid an even worse outcome. In your case, your rooster might have an eye that still has vision under that layer of pus.
You are faced with a choice. 1. Do nothing and the roo will lose his sight. 2. Risk injuring the sclera to pull the pus off and possibly save his vision.
The decision is entirely yours. We do not judge others' decisions. You know what's best for you and your rooster.