Get a decent picture, ask someone like me that sells to local pet stores if I feel they are a competent enough shop.
Much depends on the age, as well as tank decorations. It may be a bit of sparring, as angels are cichlids, it may be an injury from something in the tank, it may be the beginning of a bacterial issue.
With any of the above, you can't go wrong with some large water changes until a more definite idea is come to. Start with 50% daily for a week, every other day for the second week.