I can only guess without a photo. I imagine they are mites, a diverse group that often is found in damp environments. Many types, a few are blood sucking parasites but others not an obvious threat, feeding on decaying items, other mites etc. I take all substrates including coconut coire and leaf litter and put it in a plastic bag and let it sweat in the hot Sun and then several days in the freezer. All organic, wood products, leaf litter are suspect, so small stuff is micro-waved, and larger quantities sun baked/frozen. I probably overdo it, such is my nature... If your substrate is supporting mites, it might be a bit too damp...consider that, but I wouldn't be surprised also is some mite is adapted to even dry environments. Good luck, and investigate mite issues before taking drastic action, some folks use very toxic solutions..Vic H