I agree it looks like mites. I would spray all leaf surfaces and the medium with an insecticide soap (available at any nursery, Home Depot, Lowes). Repeat this in 5-7 days. Wait another 5-7 days and spray with an insecticide oil like Neem or Organicide. Repeat the oil after another week also. The nice thing about these two products is that they are not poisons. Still I do wear a mask so that I don't breath them. Good luck.