A dustbath area is a must have in any coop/run.
And did you look into Northern fowl mite? @TwoCrows is more of an expert than I am.
https://www.backyardchickens.com/articles/mites-lice-and-fleas-dealing-with-external-parasites.74599/
You say it isn’t scaly leg mite.
I doesn’t look like feather lice.
Red mites are not during the day on a chicken (unless you have a very bad infestation).
So I think its dirt. But not sure.
I advice you to look into some articles about lice. Like this one...