Well then... I would suspect a fish picking at them. I can see at least 2 fish that are known for nipping, especially clams. Even if you never see them do it, or even if they were model citizens in a prevous tank. I have seen angels be peacefull for 6+ years without ever bothering anything, and then , all of a sudden, start picking at everything in the tank. Some of the pigmy angels are my favorite fish, but never in my reef. Also keep an eye on that algae blenny