My pompom dropped one of his pompoms about a year ago and it has continued to live in my tank. It doesn't look anything like yours - mine actually has a double set of tentacles, one set of longer tenacles in the center, and a second set of pseudotentacles around the outside that are bulb-shaped. Yours is definitely aiptasia, unless it has any sort of skeletal structure to it, then it could be a dendro polyp. How did you get it out of your tank? Was it attached to the live rock, or glass?