Could be lymphocystis (or just a cyst caused by some sort of irritation or wound). If he's acting normally, I wouldn't worry about it. If it does happen to rupture, just make sure your water quality is great so the area won't get infected.

I don't want to lead you astray, but it could be some sort of bacterial infection that would need to be treated. But I don't recommend exposing a fish to an antibiotic when the cause of the symptom(s) is(are) unknown. So, unless someone has a specific answer for you, I'd just sit tight and keep the water pristine.

-lefty