looks like Cauliflower disease or Lymphocystis to me.
If it is, It's viral. Not treatable but many fish are naturally able to fight it off. You may want to isolate him from other fish so he doesn't pass it to anything else. Here's a link to a page that has a bunch of fish diseases including Lymphocystis. It has treatments and other things you can do to help your fish recover.
Saltwater Fish Diseases and Treatments
Bookmarks