i have a flame angel in my tank.
generally, it used to like meaty food like brain, cynarina,prata ,blasto, clams that kind.
recently tamn down a lot.
they are not reef safe and even though they do not nip now does not mean they will not in the future too.
general guideline is to feed them sufficient so that they have less chance to try the coral. a safer angel to add into a reef tank is most probably the argi angel.