Just to relate similar event recently..
I woke up other morning to find my b. tang with a foggy "bubble" formed over one eye which sent me frantically searching internet for possible problems. Dreaded thought of "disease" was first thougt -- seems to be greatest fear I have as a new tank owner.
All other fish OK.
After reading all else about "pop-eye" etc. decided to wait couple of days and see what happened.
Sure enough problem disappeared after two days.
Most likely "trauma" gotten thru her constant "sparring" matches with "arrow crab" - all other fish in tank tend to ignor crab, but the tang takes great "offense" when he tries to wave her off when she gets too close, and tries to give him a nip. Expect eye injury occured during one of these tussles.
Anyway, waiting couple of days was good advice.
Rgds,
OmarD
Tang & Crab (in background)