Do a search on "aiptasia" here and you'll find many solutions. Joe's Juice or a paste made of kalkwasser are the normal favorites. Whatever you do, do NOT manually remove it, or try to scrape it out. All the little bits will just become more of them.
I found the cheapest way to ged rid of them if you don't mind waiting for them to grow a bit, is to inject them with some lemon juice. I has worked many times for me. Just get a syringe from the local pharmacy(prepare to answer some questions as to why you need it) and a lemon. All totaling less than a $1. Poke them in the stalk quickly and accurantely and give them a good dose.