Looks like some sort of antipod / termite kind of thing. The zoanthids on the frag plug don't look any worse for wear. But I did notice some what looks like fluorescent discharge or something on the frag plugin on a couple of spots on the rock next to where the frag plug was (coral juice/blood?) But other than me moving them they look completely healthy.
Note the yellow spots on the middle of the plug to the left of the large open polyp.
Note the yellow spots on the middle of the plug to the left of the large open polyp.