If you see one aiptasia, chances there are others you don't see. If you only have one or a few consider yourself lucky, now is the time to do something.
From my experiences with peppermints they are very timid creatures, one single may not feel safe out in the open and spend much time hiding. Your mileage may vary.
What you can do today is smother them. I use epoxy putty or chewed gum blobs to COMPLETELY cover them well. It looks like hell, but it kills the damn things and then blobs can be removed later.
Cheers, pm sent