You'll be able to tell if it's lymphocystis if you look closely at it. Should look like a cluster of bubbles. If not I'd be inclined to assume it's something fungal.
If it is lymphocystis, it does go away on its own, but it takes awhile. My umbee had lymphocystis when I first got her. Took about 6 months for it to finally subside. Came back like 3 times. Nothing to worry about though, just a bit unsightly. I'd only be worried if it were around the gills or mouth, as at that point it becomes a matter of the fish potentially suffocating.