If they just crawl, at a fairly steady pace although maybe erratic in direction, they are almost certainly mites; if they jump or fly then they are definitely not mites. If that helps any.
DE *does* kill mites, however it does not kill them as effectively or thoroughly as a 'real pesticide' and will not generally wipe out an actual mite infestation (although IMO it does seem to help discourage infestations from really getting rolling). Sevin, a permethrin product, or a rotenone product are the pesticides usually chosen to handle an actual mite outbreak, being as mites can do some serious damage to birds' health and it's important to get 'em knocked down fast.
If it is mites, change the nestbox bedding (don't just knock it onto the floor, take it carefully out of the coop to the compost pile) and possibly all the coop bedding, and while things are bare nekkid use your mite treatment of choice in the nestbox and around/under the roost and where it is attached, and anywhere else accessible, *before* putting new bedding in. Then you can add more mite powder on top of the new bedding in the nestbox at least.
Good luck, hopefully it isn't mites but even if it is you can definitely deal with it,
Pat