Powdery mildew. Spray with a fungicide and let the plant completely dry out. Increase the air flow around it if you can. Don't water your geraniums so much. Geraniums like to be grown on the dry side so wait until it is completely dry and then water thoroughly and then wait until they are dry again before watering.

It may be too late for this year because I don't know what the mildew will do to the plant but next year is another year and you can take the lesson into future years. Isn't that how all of us gardeners learn.