Glass Anemones, or Aiptasia, are a real problem in reef tanks. They spread quietly, and retract quickly when reef keepers attempt to kill them. Using Kalk-paste, they can be injected until they melt away. However, all of the tissue mass must be removed or new aiptasia will return. Removal is best done outside of the tank. Peppermint Shrimp and Copperband Butterflyfish are natural predators, as are Berghia nudibranchs. Do your best to keep these pest anemones out of your tank.
Other methods include using lemon juice, Joe's Juice, Aiptasia-X, Aiptasia Rx, vinegar, or even a laser. One public aquarium caretaker would bury them in reef salt, but she was in a 550,000g aquarium and didn't have to worry about altering salinity or stinging any nearby corals with the raw salt.