Ever had a coral that suddenly would have missing tissue, even with perfect water parameters and nothing has changed in your equipments and/or husbandry? No signs of any infection and it seems to be an isolated case? And no other suspicious livestock or hitchhikers seems to be the culprit? Eunicids may be the cause:
Ngayong gabi lang yan. Isang bulate lang yan na napunit. From my friend's tank. LOL!