There is something called Lymphocystis, but since I couldn't see it clearly enough, I couldn't say for sure. It's commonly called clownfish disease. They get it, then it drops off of them all of a sudden. OR you take the clownfish out of the tank and carefully pluck it off them with tweezers. I had to do that once, and I think I was more scared than my poor little fish. He was fine thereafter.