Looks like powdery mildew to me. Serenade does wonders (or, has for me) with powdery mildew. I use it once or twice per cycle as a preventative, and haven't seen any at all since I started doing that. (It will get rid of an existing problem too). It's a bacteria that eats mold/mildew, then dies off when it's done its job.
If you decide to try it, and have a lot of vegetation to spray - get the concentrate and mix it up yourself. It's WAY cheaper that way (and, you'll have tons left over to use in the future).