Without a picture it's hard to say exactly what you're dealing with, but fungal infections are uncommon in saltwater systems. From your description, it could be lymphocystis, which is a viral infection common to angels tangs and butterflies. You often see it in newly imported fish and can be a result of shipping stress, bad water, etc. It normally appears as a white cauliflower like clump on the fins or body. If it is in fact lymph, it should clear up given good water, and a varied diet. Keep it in quarantine for now and feed it a good variety of foods including greens. Soaking foods in selcon or zoe can help. How long have you had the fish and how large is it? Any way to post a pic to be sure that's what you're up against?