Sure could be mites. They're everywhere (including the follicles of our eyelashes) and also a major contributor to pet allergies (mite *BEEP* apparently is one key component of animal dander).
If it is mites, and that's probably more likely than molds (read "molds" ~25yrs ago, but it was not a journal article, just a typical magazine article - so, how trustworthy??? Anyways, article contended this occurred after use and not initially, so, if true sounds like molds, but anyways - mites it is.)
....ok, if it is mites, then, a possible solution would be to "bomb" it.
Vets, pet stores, and supermarkets sell a very effective tri-valent bomb (several brands) - works on all arthropods:
tri-valent = a) contact poison, b) a long acting one that is carried back to "nest" (not applicable in this application), and c) a growth hormone inhibitor to break the life cycle.
Would need to greatly compress the bag and then set off the bomb and leave the area. The hope is that the expanding bag would draw some of the chemical laden air into its interior. Guess it all hangs on this last point - can enough air be drawn into the interior of the bag.
I've used these bombs in the house and no arthropods for 18mos. The gas seems to penetrate everywhere - closets, drawers, "knee walls", etc.
Would look into toxicity aspects, since one's face is going to be close to the bag - could wipe surface with a damp cloth. The last one we used, must had pretty low human/mammalian toxicity as there was no need to wash exposed home fixtures, linens or flat surfaces after "bombing", so might be 'ok'. Only food needed to be covered; not even plates, dishes, etc - even though cabinets were open during bombing. Also, label said it was NOT a problem for pets (in contact with and licking surfaces) or crawling infants. Worth looking into maybe?
Not sure how long before the mite allergens would break down though as the bombing is going to leave them in the bag. Also, not sure how the "washing" the bag Mfrs do before stuffing comes into play with removing the mites and their allergens.
Maybe more trouble than it's worth?