With mites, most people report they feel something crawling on them during/after they handle their birds.
It's good that you are keeping check on them.
To me, it looks like sand. You should be able to see Lice crawling on the skin or attached to the feathers.
Photos of lice/mites in the article below, I've found them to be helpful.
https://the-chicken-chick.com/poultry-lice-and-mites-identification/