It's not a bite. It's only stuck to him at one end, the other end is free. Might be a parasite, though which one I am not sure. Anchor worm maybe ?
Might also be some sort of tumour. Keep an eye, see if any more appear, or if his behaviour changes. I don't think it's an anchor worm, actually, but it's odd, for sure. I've seen something like it once before, never did find out what it was.