They are grazers, so they will continously nibble throughout the day. The LFS
lied to you, angels are never reef safe. Dont feed any more than normal, thatll do more damage than good. My advice, feed him flakes by dropping them on the surface of the water. While they are still floating and he is munching at the top of the water away from rock, swoop in a get trap him with a net or better yet a clear livestock holder.
Then call your LFS
and say "**** you."