Mg def? Wow, I didn't know that would do it, that is a great early warning sign I will keep in mind! I thought I had studied the effects of ca/mg backwards and forwards.
That definitely sounds like Powdery Mildew to me, sprays like that can sometimes be tough on cannabis plants, so raise your light first if you choose that product. I have a suggestion that works very well for a grower I work very closely with, it's simply called Mildew Cure and it is by SaferGro, You can get it at any hydro store. It is organic, and safe to spray on your plants up to a day before harvest. It is also very easy on plants, but lifting or dimming (if you have a sweet ballast like me) are good options for sprays like this, but it is best to spray it on a 2-3 hours before lights turn on with good ventilation. If you decide to lift your light, lift it all the way up to it's highest point for at least 2-3 hours before lowering it again.
Keeping humidity down will definitely help solve this from happening again, how is your intake and out take? Proper ventilation will keep humidity down. A dehumidifier is 70 bucks, a good inline fan is 150 bucks. If you don't plan on adding co2 (I don't remember if you said you did or not) buy the intake fan instead. If you are scrapped for cash the dehumidifier will work for now, but ultimately you are spending more, because proper ventilation controls a lot more than just things like heat and humidity, but disease and pests. It also increases yield to have your ladies dancing gently in fresh air. Always want our ladies dancing for us.