Looks like Hidden Cup Coral to me.
THANK YOU BANG GUY, WE ARE REALLY THE ONES WHO KNOW WHAT WE ARE TALKING ABOUT!!!
This is NOT a feather duster, nor an anemone... I myself have had hundreds of these things on liverock... they are baby corals of some sort... in the developing stage... Once again I say, IF it has a base that is calcium based that is attached to the rock, which btw it is very obveous if it does, since the creature is transparent, then it is INDEED a coral "baby" of some type!
Sorry if I seem a little testy, but you would be to if people were making jokes like
"are you sure your looking inside the black circle" and such...
BANG GUY, could you elaborate a little more on this subject on coral babies on rock....
Thank you!
Hope this helps!