LIKELY SICK AZURE DAMSELFISH!!! PLEASE HELP ME!!!

[Cool tangs]

Looks like damage to me, either agression or possible he got sucked up by a powerhead. Not eating will be a stress reaction. Give it time and see if there is a bully, if there is you may have to catch them, if not give the fish time to recover, try feeding frozen foods if you arnt already as fish take better to frozen foods such as mysis, copapods, etc

 

[DrZoidburg]

I agree time will be good. Looks like a flipped scale and maybe a bit of tailfin missing in picture from what I see. Similar to a bite ,crab, or power head causing it. Hard to tell from these really. Ruby rally would be a good choice if it is just a wound.