Can you just rub it off with your finger? If not, probably not mildew or mold. The plant is not old enough for notes in my opinion. Wait til she is 8 or 10 inches tall/about 2 or 3 more weeks if she isn't that tall by then. Should be fine.
If this was a western, they just got the cowboy from underneath the bull and realized his chest is crushed. The cowboy knows they can't save him. Your plants are the cowboy. You are the bull. Try and harvest it now. Make rso, shatter and or cannabutter ext.
What did you use to treat the spider mites? Could be an adverse affect from whatever treatment. If it drained heavy into the soil the roots could be affected as well.
I hate fucking mites! I got rid of some with a bit of dawn dish soap and garlic powder mixed up with water in a 1 quart spray bottle. Not during flower though.