Hitchhiker ID

[BMW Rider]

Well, I have been quite pleasantly suprised by the amount and variety of stuff that is showing up on my live rock since the cycle finished. I have been able to ID most of it as good stuff, lots of feather dusters, some pods, brittle stars and such, but this one I'm not sure of.



I was a bit concerned that it may have been the dreaded aptasia that i've heard so much (bad things) about. I have searched several sites and forums on that, and this thing does not seem to resemble any pictures or descriptions I have found of aptasia. It is a polyp of some sort, it is flat to the rock, and has ragged/frilly edges which do not seem to be like an anemones tentacles. It does react and close up when disturbed so it is a polyp for sure. It is currently slightly smaller than the size of a dime. If anyone can give me some idea what sort of critter it is - good or bad - I would be grateful.