that might be white fly infestation. try treating for that. and water the leaves daily, from the bottom.

Or is it powdery mildew, in which case frequent watering may be the opposite to what you would want to be doing. An anti-fungal, or even neem oil may be a better solution. Good airflow is the start of clearing up powdery mildew once you get it out of the location where it is doing damage.