Powdery mildew. Fairly common on the purple Acers at this time of year. As the leaves will soon fall, it is not really going to be much of a problem. We have it every year and the tree is thriving.

Be sure to spray with an oil spray in the winter, though, to kill the spores. No point in letting it take hold any earlier than you have to.

The leaves will shortly disappear, but if you really can't stand it now, and the tree's not huge, mix 1 part cow's milk to 9 parts water, and spray thoroughly with that.