Keep an eye on the two tangs for aggression but I doubt that'd be the cause. Watch where the blue sleeps at night. They have a habit of wedging themselves between things in order to hide at night. More than likely that's where the injury has occured.
I would suggest a few large water changes over the next week and be sure the foods are fortified with a good vitamin high in [acronym="Highly Unsaturated Fatty Acid"]HUFA
[/acronym]. Chances are good it will heal well enough on it's own as long as the water specs remain good.