Peppermint shrimp will not always eat aiptasia and they can also end up eating corals as well (my yellow polyps have halved in number since I put peppermint shrimp in my tank, might not be the peppermint shrimp but I suspect it is). That aside though they are pretty and I really like mine (all three will actually come out and "clean" my hand when its in the tank).
Aiptasia are generally a browny sort of colour. I have put some links in below to photos of them which should help with the ID. If you do have aiptasia then I would higly recommend getting some joes juice. I had a sudden outbreak in my tank, zapped them all with joes juice and have never seen anymore since.
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