aiptasia
This is probably a member of the family Aiptasiidae. Members of the family, including the notorious and very common Glass anemone (Aiptasia sp.), are general pests and should be cursed, maimed, scalded,...you get the picture. ;-) These anemones pack a powerful sting that can harm all those expensive corals you have in your tank. They can and will reproduce quickly in the aquarium via sexual and asexual methods. It will quickly reach plague proportions in brightly lit and heavily fed tanks. Is it easy to get rid of? No way. There are as many methods of dealing with these pests as there reef aquarists. Many have had success with using Lysmata wurdemanni, the Peppermint Shrimp. This cleaner shrimp should not be confused with another genus of shrimp sometimes known as Peppermint shrimp, Rhynchocinetes which are easily identified by their "camel" or "broken-back" apperance.. These shrimps is not reef safe and will eat coral polyps. Other methods include Opisthibranchs ("nudibranchs") like Berghia verrucicornis, injection with various substances, butterfly fish, and mutilation. Warning: Some of these methods are far from successful at controlling these pests and can actually help the creatures spread further and are added for levity :) For a more realistic approach, please see the article in the reefs.org library. Other anemones of the same family that can be found hitchhiking are Diadumene (Carribbean), Apitasiogeton (Carribbean), Bartholomea (Indo-Pac), and Anemonia (Indo-Pac). Pictures of the latter two can be found in The Reef Aquarium Vol. II by Delbeek and Sprung.