Hydroid? or pest anemone? or

[Untamedrose]

thought it was a hydroid...but as its grown...and now is showing more of itself i'm not so sure.

 

[MicroPico]

Looks to me to be a duncan coral (dotted tentacles).

 

[Charlesbrooks]

It looks like my Coral that came on my Indo rock

 

[Untamedrose]

Well I cant see the rest of the body to be 100% but the clear white spotted arms sure do match.

thanks guys haddnt thought to look into corals on this one

 

[smokin'reefer]

Looks like a cup coral. I had one and kept trying to 86 it til I posted and found an ID. Does it have a hard center? I described it like a peach pit.
Google Hidden Cup Coral.