Always hard to tell from photos but I don't see anything that would indicate ich or velvet. Could be some kind of infection or parasite that is causing the fish to flash and cause markings. My tangs occasionally do stuff like this but it usually goes away after a few days. It looks overwise healthy, how is it acting?
The way I look at it like this. What can you and what are you willing to do about it?
- If it is Ich/Velvet or something otherwise super contagious it is too late, your tank is infected. You need to pull all your fish from the tank, treat and start the whole fallow process. Or it might not be Ich/Velvet and that's a giant waste of time. Though it has some cons, waiting until more conclusive signs appear is a more measured approach.
- If it is some sort of internal injury, there really isn't anything you can do but be supportive and hope it recovers.
- If it is some sort of superficial skin damage, there isn't much you can do either. Maybe an anti-fungal or bacterial dip, but it doesn't look infected or inflamed.
If it were my fish I would (1) irrationally obsess and fret about it (2) support the fish with good conditions and (3) observe. Taking the wrong turn can be just as detrimental as waiting too long. I prefer to be deliberate when and if I choose to intervene.
