That is powdery mildew and is very common to any number of plants, usually encouraged by mild temperatures, high humidity, insufficient sunlight and overcrowding (lack of good air circulation). You can't get rid of what's already present except by removing affected foliage but you CAN keep it from spreading to clean (unaffected) foliage by spraying with some sort of fungicide. Neem oil is an organic choice but diluted milk and a baking soda solution are effective as well. You will need to repeat as necessary. And there are all manner of prepared fungicides that will work as well.

Powdery mildew control.