So,
I recently had a new setup done.
Used brand new coco-fiber, dissolved the brick, packed it down and set the enclosure to dry. After a bit over 1 week, small white bugs started "emerging" from the substrate, and they are going all around.
I'm 99% sure they can't be mites as I didn't use anything old for the setup.
I've seen something similar before, and as I remember, they disappeared as the substrate dried out. And appeared again if I would moisten it.