aiptasia X, Joe's juice, some use kalkwasser paste or boiling water injected in to them. Try to control them now or they will spread like weeds. I have one or two pop up in my tank every now and then and my tank is 15 months old. I use Aiptasia X. Some folks might suggest adding livestock that eats them, such as peppermint shrimp, copperband butterfly fish, berghia nudibranchs. Unfortunately the fish and shrimp are hit and miss and the nudibranchs will die after all the aiptasia is gone.