Do you only have one? And if so, is it on a rock that you can take out of the tank? I recommend treating it outside of your main tank, perhaps in a bucket, so that, just in case it releases any spores, you won't be getting it all over your tank. I've had great success with Aiptasia-X, which you should be able to find in your LFS. Follow the directions carefully. Keep that rock out of your tank for at least a week and watch it carefully for any other aiptasia that have been in hiding. Once you return the rock to your tank. invest in some peppermint shrimp. They like to eat smaller aiptasias and are good to have in case you missed any.