Hard to say. I agree a microscope will be needed to identify. If the mite is crawling around fast and doesn't get on upper leaves it most likely is Hypoaspis Miles. These are the good guys that eat things like Fungus Gnats, Spider Mites, ect. ect. After looking at your grow, I can tell you're most likely using dry amendments and doing a "Living Soil" style of growing. More than likely these are hypoaspis miles. Your plants look like they are in great health. How many do you see crawling around?
I use a living soil method too and always have them pop up. TONS of them.