Search up how to make kalk paste, its easy to make and inexpensive and does a good job of killing them. Just do it ASAP!
IMO you should take the rock out and put it in a small tank or anything filled with tank water until you have eradicated the aiptasia with kalk paste and then put it back into your tank.
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You will find useful information about possibilities to eliminate Aipatasia anemones.
If you use Kalk or Vinegar, be careful with the amount used as it may unbalance your water parameters and avoid spilling on neighbour corals, so disconnecting the pumps while performing the "treatment" and wait some 10 mins after it its done before reconnecting the pumps again, is a good and advisable action.
If possible it is safer to "treat" the rocks out of the tank and for that you can find detailed information on how to do it in the link above.