theres a bunch of different kinds. the most invasive is the blue clove polyp. i had some similar to those. they didnt mat like the pretty metallics or giant cloves/ they quite literally disappeared into the rock. no positive ID just anecdotal stories.
I'll keep the thread updated with what these do, they seem to have come up from under the Hawaiian Pocillopora... perhaps it was on the plug and I didn't see it. As long as it grows on empty rock and not over any of my corals I don't mind. So far they are growing away from the coral, not on it.