Hydrangea - powdery mildew?

Looks like it. Dryness is the usual cause.

Baking soda has been proved by many gardeners to be effective in treating powdery mildew. Mix 1 teaspoon baking soda in 1 quart of water. Spray plants thoroughly, as the solution will only kill fungus that it comes into contact with.
Milk spray is another effective home remedy. Dilute the milk with water (typically 1:10) and spray on plants at the first sign of infection, or as a preventative measure.
Once plants are heavily infected, it’s very difficult to get rid of the disease, so focus on preventing it from spreading to other plants. Remove all infected foliage, stems, and fruit and destroy them, either by throwing them in the trash or by burning. Do not compost any infected plant, as the disease can still be spread by the wind and persist in the composted materials.
Sterilise pruning shears afterwards.