Was it always there? And, do there appear to be more of it as time progresses?
A lot of people say salt baths will help if the fish sick so even if you don't know what it is maybe try doing them for your fishy?
I only say this because if your fish does have a problem, and I don't know enough to say he does, it will be harder and take longer to get better the longer the time period it goes without treatment. Besides even if your fish is not sick they say it helps with overall health and fin repair