He might not eat all of your zoas, and he might not eat any at the beginning. But odds are that one day when he/she is mature they will start munching. Maybe just a small patch here, and a small patch there. And then leave the rest alone for awhile.
Or sometimes they just start and don't stop.
With the big Angels, the chances of them leaving the Zoas alone is slim -- Possible but slim.
I have kept big angels and sometimes they will be fine for months, and then one day take a colony out of the blue.
When my tank has been doing well, I have 1000's of heads of Zoa. I once raised a Chrysurus and all of a sudden one day it mowed 300+ Bright red zoas. And a week later some blues. And then it kept going.
I have kept Imperators and had the same experience, of them getting an occassional taste. Not decimating the entire zoa popuplation but just one color morph at a time.
Dave B