This is the second time I've been able to witness this behavior and still I'm dumbfounded. The first time it was with my Christmas Tree Favia and last night it was with my Rainbow Acan (no longer rainbow colored)
At first I thought they were actually eating the bristle worm. However last night I watched the entire exchange and still am clueless as to what is happening. The bristle worm appears like it's being eaten but then slowly the acan released it and it slowly tumbled to the sand bed. Showing very little movement for a bristle worm. This morning it was gone. Whether dead or not I can't say.