If it's in the tank it's likely too late... "The reproduction in these species are well researched. The Aiptasia belong to one of two genders. The two genders release their gametes into the water where the fertilization occurs. The resulting zygote turns into free swimming planula larva. These free swimming planula larva flows with current for a while before they settle down on a hard substrate.".
Even after removing the rock with the offending species you will probably still see others start to pop up. Been there myself, which is why I started QT'ing even LR.