Very bad. All dwarf angels are notorious as being coral nippers. I had a bi-color and it nearly destroyed my tank. I was in denial for several months, but I finally decided to tear the tank apart to get him out. The very next day the tank looked so much better that even my wife commented and asked what I had done. The issue was that the fish didn't visibly seem to nip that much, but my corals' growth and expansion was improved dramatically overnight. The very next day I could see tons of feather dusters that had been hidden due to the picking.
Some specimines may be safe, others definitely are not. If you are willing to take a chance, and willing to later tear your tank apart to get him, go ahead.
At the time, I didn't have any clams or SPS. I have heard that they will nip at clams ans at SPS more than softies, so if you have them or hope to get them I would seriously reconsider.
Ernie