It looks like a worm (a feather duster) to me. They are tiny (2 mm across the circular mouth) and they populate one of my live rocks. Here is a pic (excuse the quality - they are really tiny):
Sound like a possible description for glass anemones. Are they like tentacles that are more clear than white and arranged in a circle? Do they completely contract back into hole in the rock? Typically feather dusters have rows of circles of feather like tentacles where as glass anemones have just one row of tentacles in a circle.
I just saw something very small like this growing out of a dead snail's shell. If I don't have any coral should I worry about it. Could it be a feather duster?
These don't seem to spread noticeably and they have never grown bigger than what you see on the picture. I have one peppermint shrimp in the tank that enjoys exploring these rocks and may control the population of these hitchhikers.
I agree, most likely harmless tube worms. For more info and photos please see the worm section of my hitch hiker pages (link in signature). Hope it is of use.
It turns out they really are worms. They live in tiny rigid tubes and will retract to them when scared. A couple are big enough to observe this but I just couldn't take a good picture.