Is this the only one you see? Berghia Nudibranchs to remove, the real ones. Give the nudis away to someone who needs them after your aptasia is eradicated. These are considered an aggressive pest that will take over tanks, if not addressed.
If its only one and you can remove the rock easily I would use either a soldering iron if you have one or heat a screwdriver up on a flame, remove the rock taking note of the exact place it is, remove screwdriver off the flame and jam it in its little green face, makes sure you get right in the hole its retreated into, I then heat my screwdriver up again and repeat once more, rinse off in old tank water and put back in your tank, it's always worked for me doing it that way.
If you can't remove the rock then you'll have to do some in tank treatment, on the UK forum we have something called death juice that one of our clever scientisty blokes came up with, I won't post the link though because a) I might not be allowed to link another forum and b) even though it's made from chemicals readily available it's dangerous nasty stuff and maybe it's against your laws