Possibly the angel. They can decide to snack on corals...What are your levels? Have you introduced anything new into the tank? Was that coral there for a while, or is it a new addition? If new, how did you acclimate? If everything else checks out fine, watch your tank for a little while to see if the angel is nipping at it. If so, that will give you your answer right there. But first things first, check your levels!