Ok, stop worrying, it dosent sound like mites of lice to me. It sounds like a very normal partial moult. Hens go through this, sometimes several times a year. If you look at the new feather quils coming through, they are surrounded by a little waxy sleeve/tube. When the girls preen themselves or the feathers grow this breaks or crumbles off leaving what looks like large dandruff flakes. Have a look at their new feathers, you can slide or break this covering off yourself without hurting them.Moulting can take it out of them a bit, so keep an eye on them, maybe pop some poultry spice in their food or life guard tonic in their water just to give them a bit of a boost.
If they have lice, chances are they would be crawling on you too, you should be able to see them or their eggs around the base of their feathers. If you did find them, you need ivermectin , or a lot of people on here use kitten strength frontline, although this hasnt been registered for poultry.Try googling moults in chickens and lice/mites. Also use the search engine on here, there have been some good posts with lots of experianced members giving fab advice.Good luck
