Sounds like lice. Permethrin garden dust or permethrin horse spray is a available. Check your others for lice. Treat once, and again in 10 days to get the eggs that have hatched. Coop,roosts, and nests, should be treated and bedding replaced. Here is a good link to read:
https://the-chicken-chick.com/poultry-lice-and-mites-identification/
https://the-chicken-chick.com/poultry-lice-and-mites-identification/