I can think of two things that it could be. First, it does resemble an aiptasia anemone. Six known species. They love reef environments. They reproduce asexually buy budding. According to the magazine Marine Fish and Reef 2004 Annual you really don't want them if that is what they are. Second, at first glance I thought of some sort of a feather duster which are not harmless at all. They are a neat addition to a tank. My parents and I both have very small ones in our tanks from the live rock.