It is not aiptasia.
I found one of those guys at the LFS while I was working. It was in a large skimmer, on the inside below the collection cup. I was interested so I took it out and put it in my nano at home. After placing it in a low flow area and seeing it attach to a rock, I let it be. It only extended in the dark and retracted to a ball during in the light. The next I couldn't find it anywhere. I later found it on the underside of my monti cap, red polyps extended and looking happy.
Because of it's ability to move I'm assuming it's some sort of anemone. What kind, I have no idea. I'm calling it a non-nusiance, since it doesn't sting anyone and hasn't reproduced all over the tank. It is definately not aiptasia, though.