There is a couple of things it could be. If it’s fuzzy, it would most likely be a fungal infection which is easily treatable with API fungal cure. Or, it could be Ich, which is a lot worse and you’d need to separate him immediately. Usually ich is peppered all over the body though. 3rd, it could simply be a small cut. You’d have to post a picture. To post a pic, when you go to post, hit the little camera in the bottom left. I use my phone.