Are you sure something ate them and they didn't just melt? Sometimes, especially with wild collected yumas, and I have seen it on colony rocks more times than I want to recall, they will melt away due to inabiltiy to acclimate to tank conditions. I personally suspect that yumas, being a deep water coral, do not fair well if exposed to air, of which they are in most cases when collected, transported and transfered to most LRS.
I have seen hermits chow on dying corals, rics included, so maybe the stars were doing a clean-up.