Thank you for the post and observation. I've seen what you describe, and further, understand that attachment sites for tridacnidae clam larvae often is the gouge in a coral face made by a large parrot. However, the parrot is after the algae (zooxanthellae) in the tissue of the coral, more than the coral, itself, or so I understand. Unlike the tangs, who go off the reef to feed, the parrots mostly remain on the reef with little access to plant matter. With proper feeding, heavy in vegetable matter, the coral may be mostly safe. Perhaps a fish only tank might be safest.