If you can pull that rock out and QT the whole lot of aptasia, I would do it immediately. There are a lot of ways to kill them off. Aptasia X and other brands , kalkwasser, hot / warm water as mentioned so you can pull them off - which makes me nervous because anything left behind will regrow, lasers, etc. There are also predators for them - I'm sure you can find a ton of info here on the forum. Aptasia will sting your corals and reproduce like wild fire.