Trying to ID

[Digicop]

This rock has been in almost complete shade for the last several months. When I moved it out I observed these on it and had no idea what they are.

 

[Maritimer]

Those look like ball anemones - Corynactis, IIRC.

Mostly harmless, though there's a larger (much) species that goes by the same common name, and can be pretty dangerous.

~B.

 

[JGT]

Ball anemone, they are fine in a reef tank.

 

[ReeferWarrant]

Ball Anemone