I planted a small Mist Maiden this spring. I have noticed a white substance covering the leaves. Is this powdery mildew and if so, do I have to treat it?
Its sounds possible; a picture of the problem might help confirm powdery mildew. Here is a list of controls you can use against powdery mildew from easiest/non-toxic to chemicals: avoid dry-wet-dry-wet soil conditions; aim for evenly moist conditions instead; good air flow in between plants will minimize the chance of PM too so do not plant shrubs close to each other; pick up plant debris and throw it in the trash when you have PM or other fungal issues; apply the Cornell Formula containing baking soda and water, or apply 1 part milk to 9 parts water as milk can act as a fungicide (increase the milk-to-water ratio when you have lots of PM); Neem oil or all-season horticultural oils. If you want to use chemical fungicides, the Alabama Cooperative Extension Service recommends those containing these active ingredients: azoxystrobin (Heritage), fenarimol (Rubigan), parafinnic oil (Sun-spray Ultra-fine Oil) and thiophanate-methyl (Cleary's 3336). All have to be re-applied until the infestation is brought under control but Heritage can be re-applied the longest nbr of days (every 14-28 days... versus either every 7-10 or every 10-14 days for the others). See the link below for pictures of leaf problems.
Here is a link that might be useful: Leaf Diseases of Hydrangea