Hmm...I've never seen mites like these that stayed just on the pots...
I HAVE, however, had mold grow on some of my terracotta pots that my plastic ones are set in. Ranged from white, to green, to dark green/brown.
For now, I'd wash off what you can until you can get more definitive answers.
Maybe try soaking in a bleach solution (if it's mold, 1 tsp bleach to gal. water) or very mild soapy water. (you can probably scrub the pot itself with regular soap, but weaken it if you soak the plant)
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