It looks like mites. Do you see any crawling on the chicken? Permethrin 10 spray or garden dust would be good to use. Ivermectin is also good to use. Mites can reproduce every 5-10 days depending on the type, so repeating your treatment every 7 days to kill the mites plus the newly hatched ones before they reproduce. Also, your coop, nests, and roosts, need to be treated with the spray after removing the bedding and either burning it or bagging it.