looks kinda like the start of powery mildew. If it is it will move fast. I have never dealt with it personally. Your stats seem to indicate prime growing and breeding conditions for it. High humidity, and moderate heat. Is your PH rather acidic? How is your airflow?
If it is mildew. raising your PH to run more alkaline can stop the reproduction as well as decreasing your humidity and increased airflow. I know you got to hit it hard in all three of the areas for it to actually be erradicated.
Good luck. Hopefullly someone else can chime in.
