If you remove the rock from your tank, and make sure to remove all bits, it will work. Rinsing should flush away fragments, and prevent it from coming back. Dosing with kalkwasser works as well, but you run the risk of a chunk floating away--and this chunk will grow into another one.
Aiptasia are very bad. They will sting corals and fish, and spread like a weed. Worse than a dandelion, IMO.