Difficult one to guess at Demonfarmer but I'll have a go.
Red mite are usually in the coop, but they are red when fed. They will travel on the birds in very small numbers and when found there they will be red. But if they are from hatched eggs they would appear white or pale brown until they get a feed. They could be from eggs, but they take about a week from being laid in cracks and crevices to hatching. I don't know where your hens are now, but if they came from a coop outside it will be total infested. To get them off the birds try the Barrier red mite powder -won't kill them but just gets them off so that the birds don't carry them back into any new or treated coop. The mite are about the size of a pinhead and killing them can range from simple diatom or potash (which you can add to the soil bath areas), through some simple chemicals like Smite or Nettex Mite Kill, creosote of the coop, to a biocide like Elector which you spray everywhere and is very effective for one year only. Before any of that I would get a steam cleaner with a fine jet and run that around. That is our first approach-it reduces the number dramatically.
Northern Fowl Mite live and feed under the skin of the birds, usually around the vent but certainly not always. They are microscopic and rarely large enough to see. A very damaging pest killed effectively by spraying the areas with Frontline at 0.5 -1.5mL per Kg. Any more and not only are you wasting it but you may kill the bird. It stays in the skin for many months in the sprayed areas so future attacks will probably be in totally different areas on the bird.
Lice take many forms and range from a few 2mm long to about 6mm in my experience. They only live on the birds so get them off and they die. A simple application of Barrier Louse powder works every time. Make sure of the wind direction when applying as you don't want to get it in their eyes.
So my guess Demonfarmer is that they are red mite hatched from eggs actually dropped off the birds, something I have never seen before. However it may be some pest that I have never encountered before, so good luck.