I beleave what you have is called Aiptasia. There are a few different was to combat these things. I take a syringe (you can get one form a local drug store) w/the largest needle thay have and inject the suckers with white venager. Some folks use a very strong mixture of kalk past. Or a combo of the two. I do 3 - 4 doses full a day of just the venager.
Peppermint shrimp should take care of the small ones for you.