Or possibly a sting from an anemone or coral. Some things are easier to spot under blue lights, but by the time they get through a camera, they're often very difficult to identify.
Looks like an injury that was swollen for a bit. It's likely to heal this up on it's own, but keep an eye on that little red spot. If that doesn't go away in another day or so, or gets bigger, then you'll want to remove to QT for antibiotic treatment.