YOU HAVE AIPTASIA!!!! >:( Sort it soon, they breed like rabbits. :(
Go to the forum 'Learner's Room' where everyone gave me advice on this. it was really helpful too. ;D
I have had a nightmare with them. I started by giving them a paste of Kalkwasser, didn't do much, I couldn't source Peppermint Shrimp in my area, which a lot of people use to control them, and didn't want to put any unnatural stuff in, such as a product called Aiptasia-X, which is reef safe, but apparently not too helpful in preventing a large outbreak. :-\
I finally went with Berghia Nudibranches; rather expensive - especially here in the UK -as they only eat aiptasia/glass anemone. They have been in the tank for a few days now, and I am waiting for the 'feeding frenzy' to start.
Don't try to just take them out of the tank, they will multiply when threatened!!!
I currently have about 20 or so!!!