Hey Growmie,
This is exactly what you said .... White Powdery Mildew, good thing you spotted it early. There are quite a few products and homemade concoctions people use to treat White Powdery Mildew. Among the effective treatments are:
Milk (1:9 ratio of milk to water)
Baking soda (2 tablespoons per gallon of water)
Neem Oil (4 teaspoons per gallon of water)
Hydrogen Peroxide (1 teaspoon of 35% H202 per gallon of water)
SM-90 (1:5 ratio of SM-90 to water)
Hopefully, I answered your question and remember Happy Growing and Free your mind one puff at a time.