Hi there, welcome to BYC!
I was thinking maybe grain mites.. but I've never heard of being bitten by a grain mite.. so I might think.. another type of mite that's still making it's way to the chicken that maybe came in on the bedding.. I know some folks complain it's the number one reason they avoid using straw hay.. I haven't had that issue YET.
I would consider using a permethrin based "horse fly spray" on all the coop surfaces, lay boxes, under roost, walls, etc.. found in the equine section sold under many brand names and different concentrations from ready to use spritz bottle or mix in a garden sprayer..very affordable and effective.. while also being safe (not for cats) and requires NO egg withdrawal time WHEN used on birds.
Noting that one of our members took a mite into their county agriculture department to get identified.. to which she was handed a thick book and told to have a look.. there are thousands of varieties! My recent battle with fleas has revealed there are over 2,200 flea species.
I do notice they are on your water thing in this photo.. one of my revelations was that they actually require a high amount of humidity to survive. It's possible they are a seasonal pest.. flies are OFF the hook right now, here.
FWIW.. a little dish soap water on a wash cloth might take care of all the ones you can easily access. Standard water, will not likely have any effect.
Consider adding your general location to your profile, as it may help folks make their best suggestions possible at a glance as many things are heavily location dependent.