Hard to tell from the pic, but probably just a small anemone of some sort, maybe a cornyactic spp. Like aips, or any other small anemome they are only a problem if they get out of control. The cornyactis that I have in my tank are mostly nocturnal and don't cause any probs, and I have lots of them. But I know David had a similar looking anemone in his sandbed that caused lots of probs.
Dan