I have no aiptasia, my peppermint (singular) ate them all from my 90 gallon tank. 20 shrimp are gonna get hungry and that is why they probably hit the feather dusters. I have tons of small feather dusters all over my rock and my peppermint shrimp doesn't touch em. This is the 3rd pep I have owned (the others became meals to a SFE that decided he prefered them to frozen food) and I have had no problems with them.
Aiptasia can harm corals and fish when they get bigger (2+ inches) so I would recommend getting rid of them. A pep might avoid the larger ones so I would recommend the solutions to kill them.
Aiptasia have a way of popping up on LR even after you think they are all gone. I have had some pop up months after I thot they were all gone and the shrimps had been eaten
. Keeping one around (pep) has insured they (aiptasia) do not return - and they are cool to watch when they come out of hiding.