At this point, I could be wrong, so you might wait 'til it opens. May just be a zoanthid or a sponge or something. Besides, pulling it off may leave a little blob that will come back to haunt you.
looks like you already juiced it, sometimes thats what they do when you try to kill them. I have about 3-5 in my tank that did that and are still like that 3weeks after the fact.