if there is one on his shell, lift it out of water and use a knife to scrape it off the hard surface. Dont let them exist in your setup
its possible the pet store live rock may get some, lift out, set on counter, and dig out with a knife then rinse the area off with saltwater and set back.
if you search out aiptasia threads you’ll see keepers using every other means aside from the direct one, and there are lots of aiptasia takedown threads too as a result. It’s just like cycling: the most common approach isn’t always the right one, work threads show.
you will see that pico reefs are very rarely if ever challenged by aiptasia, and large reefs routinely are
the difference is willingness to access the rocks outside the tank for direct removal