This looks like powdery mildew on the foliage. In general, powdery mildew is a common late season fungal disease. It is not usually a big problem on Japanese maples. Apparently the weather conditions were perfect this year. 

There is nothing you can do at this time. When the leaves fall, you can collect them and discard them in your trash to minimize fungal spores overwintering in the area. If mildew starts to become a problem next year, you can decide if you want to manage it with horticultural oil or tolerate it. Powdery mildew can be unsightly but usually doesn't cause lasting or serious damage. Here is more information on the disease, with ideas of how to prevent problems in the future. You may not have an issue next year.  https://extension.umd.edu/resource/powdery-mildew-disease-trees-and-shrubs

Marian

Thanks very much. You folks are very helpful as I ask lots of questions. 

You are welcome.

Marian