What pest has my new hen got please

Hi my name is Mue and I have taken in some hens as the owner passed away.
I need some help with these two new hens which have just settled in after two days and I can hold them. They have come with something in their feathers. The hens are white and the other dark brown. The white hen has white thin longish things in the feathers that move not fast or these didn't. When I found it this morning I did the following:
1. Burned the bedding from their coop (even though I did put lots of SMITE powder in the bedding and cracks before they came two days ago.

2. Squirt SMITE into every crevice again and dust the bottom with SMITE. Put nedz bedz bedding in a lots of SMITE on top. let it settle. Then gently raked it into the bedding and nest boxes and perch. Well everything got covered in it.
3. Hold each hen as I carefully went under the feathers and put SMITE powder all over the hen apart from the face and vent.

I think they have been wormed. My four hens are not with these but I do the same with them. They have no bugs and have been wormed.

Does anyone know what they are? How I treat them? How long for?

If I knew the name of this crawly thing can you recommend a treatment. The white bird is laying eggs but not the brown one. Unsure of age but mature birds. lovely looking feathers.

Thank you in advance please

Mue