Is this some type of disease or did my firefish get injured?

[fronty]

I noticed some damage to the back side of my firefish. It looks like a piece of its fin is missing and there is an abrasion or white spot. Here are some pics:

Livestock:

Pistol shrimp
clown fish
yellow watchman goby
emerald crab

 

[Newsmyrna80]

His tail looks like a crab nipped at him. My firefish has this occasionally. How long has he had the anomaly on his side? Is he eating and breathing ok? Is shaking his head?

 

[snorvich]

Are there coral banded shrimp in that tank?

 

[fronty]

I just noticed it today. I turned the lights off last night (no moonlight). That may have been a contributing factor. There is a pistol shrimp and YWG pair that took his hiding place in the rocks. Other than that, there is an emerald crab (small), and a small clownfish. The clownfish doesn't seem to bother the firefish, but the firefish does try to avoid him.