I agree on Lymphocystis which is a common viral infection that affects the skin and fins of saltwater fish, mainly angels and butterflies. It does not cause any health problems, and rarely if ever spreads and may disfigures the fish.
It gets its' name as it causes growths on the skin or fins, which look similar to cauliflowers. as mentioned, it does not pose any health problem to the fish, but the growths may make the animal less aesthetically pleasing.
Really no treatment recommended as it should fall off by itself. You use melafix which will protect rest of fish and is reef safe.