Don't freak out. This is a common nuisance, and it actually helps to clean the tankwater. They tend to sting and kill corals, though, so they're better off in a sump.
Squirt really hot tapwater at it and that should kill it. Get a Peppermint shrimp to take care of any smaller aipstasia that may have spread.
If your tank is big enough, you could put a Copperband butterfly in there too. I think that's the type of fish that eats aipstasia.
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Halfbaked