What is this?

[fishytim]

I was looking in my 20 gallon hospital tank today where there is one bigger peice of live rock and I see these two flower looking things. At first I just thought it was some kind of algea because I just bought this tank from a guy and the tank is rather dirty (mostly green algea and diatoms). Anyway I put my head real close to the tank and told my fiance to come look at these things that had popped up overnight and when i spoke both of these little flower looking things closed up and went into the rock...
Anyone have any idea what these are?:help:

 

[viper_930]

Were they white colored? Probably two little dusters from a duster cluster. Harmless filter feeders that usually come on LR.