Clown fish falling apart

[boxer rox]

I have an Ocellaris clown that has a semi circle spot missing on its back, right in front of its dorsal fin. The fish is only about 1 inch long so its hard to see very well but there is a half moon piece missing from its back. It started out with what looked like a piece of white raw tissue on its back and that is now gone but the piece is missing. It looks like something took a bite out of its back. This is a 30 Gal tank and the other fish is a Royal Gamma Basslet. They were both added at the same time and really pay no attention to each other. There is also a Peppermint shrimp and several Hermits and Snails. The Clown is still eating and never stops swimming however it does go to the surface sometimes and once in a while will swim vertically(standing on its head). Do you think it has a disease or would any of the other residents have cause to take a bite out of it? I have searched the many diseases and cant really find anything that matches this. I'm fairly new to this and any help would be greatly appreciated. Thanks in advance.