Looks like a coral. I would think Clavularia but the polyps seem too far spread out. Are there any runners connecting the polyps? Do the polyps retract into the rock when disturbed?
. No matting, or beds connecting them. but all over the rock. seems to being growing directly out of the rock. So At first I assumed a cluster of feather dusters. When disturbed they dont full retract into rock.
I'm beginning to believe those are surviving coral polyps from whatever coral skeleton your Zoanthids are on. That's pretty cool. I do not know what that coral would be though.