Actually after searching what you wrote, I dont think that is what it is. I could be wrong I am some what of a Novice. I was thinkin more along the lines of some type of polyp.
I also agree ithat it does'nt look like aptasia as the "arms" are too think and short. Also in agreement it looks more like a zooanthid, but how will feeding it "tell for sure"? a zooanthid will also eat a meaty offering.
A piece of food will stick tight to an Aiptasia tentacle and the Aiptasia will quickly yank the piece into its mouth and curl up to consume its catch, sometimes lightning fast.
A zoanthid will barely be able to hang onto a piece of food, if at all, and will slowly curl around it, sometimes so slowly that you can't even see the movement.
Well I droped some Phyto Plankton into the tank and the, What I will call it polyp, slowly closed its edges around it. It happend pretty slow but was pretty cool to watch. I still say it is deffinetly not a Anenome