My digitate acropora don't get touched by my emperator. It will nip off growth tips on a lot of acros but won't and probably can't eat the thick Nubby corals. Anything with skinny branches is fair game. It will take the tips off of healthy colonies every week or two but they grow back in days. It doesn't prefer a certain one just which ever one it feel like eating tips off of it seems. Validia, abrolhosensis, Millie's, and anything with a nice plump edible tip it can fit its jaws over.
I have a tank with colonies and lots of frags. It will not eat the tips off of single frags, at least has not yet. We keep it very well fed. I am not sure why it hasnt eaten frags, maybe there is something in the colonies new growth that it likes thst isnt in a fragment of a coral. I would guess its the large polyp on the tip. Angels we had that don't eat corals at all (but can and will if not fed or housed properly) are, African flameback, flame angel, and most genicanthus.
Our multicolor would mouth certain favorite acros to get the slime coat off and rub flesh off. This was the most destructive type I have kept. And the only one that actually seemed to prefer certain corals. In this case the favorite was a yellow tip austera.