Those feather dusters shouldn't be killing your Zoas, they are just a filter feeder. If anything they might irritate some polyps but not kill them. Have you checked for other pests?
You dipped your coral in freshwater? That killed a lot of good things on that rock that's necessary. Many things will kill zoas, it's not a feather duster that will.
I would keep an eye on them for the usual suspects but also make sure your parameters are in check! I agree with above, a fresh water dip is not necessary.