Could be fungal or possibley lympho. In either case the best treatment is being sure of the highest water quality and that all foods are fortified with a good marine liquid vitamin high in [acronym="Highly Unsaturated Fatty Acid"]HUFA
[/acronym]. A few good water changes with well aged SW
will reduce any DOC
in the system feeding the problem and the vitamin will boost the immune system of the fish allowing it to fight the problem naturally. Ocean nutrition brand Formula 1,2 or Prime reef foods would be a good option to add in the mix.
After a short time you should see a reduction in size if you continue to make sure the water changes and feedings are maintained. If a true fungus, you may end up needing to treat in a QT
tank with an antibiotic but I would do that as a last step if there is no improvement.