No, it's not aiptasia. I have one and have had it for years. It was a hitchhiker on live rock. I can't recall the species name, but I've seen an id before for it. They cause no problems, but are very sticky. They seem to like to attach similar to a BTA in the rockwork. I even occasionally feed mine, which is quite a spectacle. They immediately clamp around the food with their tentacles and pull back into the hole they reside in.