No that is not substrate, those are mites filled with fish flakes. I leave a small bowl of fish flakes in for my hissers and after 2 days this is what I see. I really wish I could get some earwigs to feed them to.
I have become far more tolerant of mites in my centipede enclosures, and simply accept them as I do springtails, but seeing a swarm in one of my roach colonies is a different matter. My dubias are especially bad as every death feeds and army.