It looks like a tumor or cyst to me, tumors in fish are untreatable (but generally not infectious towards other fish, as they tend to be genetic), if its a cyst though you can use an anti internal and external bacterial med to get rid of it. Another posibility would be cauliflower desease, a "viral infection that causes cells to become megaloblastic, thus forming small tumors (bumps or growths), often along the lateral line or the pedicle, where the fin meets the body. The tumors sometimes take on the appearence of tiny cauliflowers, thus the name";
http
/www.fishpalace.org/Disease.html#Lymphocystis
Personally, it looks like a tumor more than a cyst to me, i'm no expert on marine fish though so this could be a physical ailment that effects only your particular types of marine fish etc. But for now, i'd say its either a tumor or a cyst.