The bees do seem to leave behind a lot of debris as well, my fading peekers struggle to differentiate between them and dead mites.
You dont say if its a solid floor or OMF with a card below it. Either way are the mites dead? If so no problem in themselves but could be indicative of a bigger problem if varroa - probably wise to start an apigard/Apivar life/thymol type treatment as soon as if not done so already.
Sure others, more knowledgable, will be able to add or correct this