Looks like Aptasia to me, and yes, they are a pest. :scared: There are lots of theories on how to get rid of them: Kalkwaser, boiling water, Joes Juice, Pepermint Shrimp, etc....
For the larger ones, I'd suggest getting some Joes Juice. It's not too expensive, but it does the trick.
If you have a lot of them and they are small, I'd get a couple of Pepermint Shrimp and they will do the dirty work. My Peppermints cleaned up the ones that managed to evade my Joes Juice efforts.
either way, this won't be solved over night. You just have to be more stubborn than they are.