The best way to probably get rid of it (which you need to do) would be to use joe's juice. It works very well. What you do is turn off your pumps so the water is stagnant then use the syringe with joe's juice in it and feed it to the apstasia then like 10 minutes later suction the dissolved aptasia and you should be good. or if it's on rubble I guess u could take the rocks out and leave them outside in the sun or something for a day to kill them.
That's a pretty cool idea to set up a small tank just for it. It's like how some people set up small tanks to keep mantis shrimp.