little anemones? hard to tell from the pic, but it sounds like aiptasia. They're a pest, and peppermint shrimp are a great natural way to get rid of them! I believe they get invasive, and I hope someone else chimes in.
Here, I did a search of the forums, just click the link and browse those posts, I'm sure you'll get a better idea of them.
I think they are tunicates because of the single vent. There has been some confusion as to what they are. They are good (but I actually have one growing on my black cuccumber). I did see a refeance page that called them sponges, but I think they may be a tunicate that has sponge in their name kinda like cardinal fish its a fish not a cardinal.
well... i wouldn't call them a bad thing... but i can't say they are good either. the more i look at them the more i realize that having them means your tank has enough dissolved solids and floating proteins(phosphate) to keep them alive. you may want to look at beefing up your skimmer to remove more pollutants.
they won't hurt your tank... but not having them is a good thing too.
just my .02
I have a bunch of them on my live rock. They kind of freaked me out when I first seen them in my tank. They have been falling off my live rock and there aren't as many in my tank now... maybe I need to check the filters. hee hee