Red Firefish with a bruise?

[Saiyan]

Hey guys can I tap into your extensive knowledge for a minute.

What is this?

It looks to me like an injury but I am not sure. Can anybody ID what is wrong. There is a lump right in front of the red mark. He is eating fine and swimming normal. The only symptom is the mark and bump. Could this have been caused by a pistol shrimp or a burn from a coral? This is my favorite fish any help would be appreciated.

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