its a form of aptasia anenome,
I actually purposely keep 3 of those in my tank....i've had them for 3 month and so far they haven't grown or split. There were several times i was pretty sure they were splitting, but none yet.
They have acutally been well behaved, all three are on the original rock, and have not yet moved off the rock. There are also some leathers on the same rock, and the aptasias have not stung / killed any yet, at least not that i can tell.
If only i could get my BTA's to be so well behaved, my BTA's keep running around every one to two weeks.