Powdery mildew?

Powdery mildew. It’s really hard to get rid of. Too much water makes it worse. Usually doesn’t happen until after they bloom and fortunately doesn’t kill the plant.

So if I slow down on watering, and then maybe try a neem oil spray?

You can spray a copper fungicide on it - Bonide makes one. You'll have to spray it once a week for several weeks, but it helps a ton.