I have also had a few different varieties of angel (koran, flame, bi color, and coral beauty). The fish never made a real noticable impact on the corals by picking on them but they still did. I know I have sat on numerous occasions and just watch the angel bite at the corals. I have seen my koran pick on zoanthids, leathers, and kenyan tree. I did notice that if I kept all the fish "fat and sassy" with food that they didn't pick on things as much. I do believe that it is their instinct to "graze" on the liverock and pick at the food available.
Brandon