In the wild, Angel's graze for food, this is why they can be not reef safe. Some, like a flame angel may just be happy with the food you feed them but if their grazing instinct kicks in they are going to eat your polyps real quick. Sometimes they just change on you too, they don't touch them for years then YUM YUM Zoas! In my experience you're always rolling the dice. Some fish have a lower % of eating coral, clowns and tangs for instance. Some higher, butterflys and Angels. That being said I have a Trigger that doesn't touch coral and a Yellow Tang that snacks on a pavona cup coral.
You never really know, you're just reducing the risk.