Poor air circulation, low light and cool temperature are some conditions for the development of powdery mildew which usually affects the young growth on a plant.  Ensure the plant gets good air circulation, bright indirect light and temps not too cool.  When watering make sure the water doesn't splash on to the leaves.  A non toxic control for mildew is baking soda combined with a lightweight horticultural oil.  This can be applied to the affected leaves.  Suggest you test on a few leaves first.