Since it is relatively late in the season (and the leaves have done their job producing food for the tree), it shouldn't do much damage as the leaves will probably fall soon.    After the leaves fall, try to clean these up and dispose of them so the leaves won't provide an overwintering spot for the powdery mildew spores.

Although these powdery mildew spores are primarily wind disseminated, you might also consider some pruning of the tree to improve air circulation (this can help make the tree a less favorable spot for fungus to thrive).  

Here also is some information from the Pacific Northwest Plant Disease Management Handbook - https://pnwhandbooks.org/plantdisease/host-disease/maple-acer-spp-powdery-mildew.    Next year, in late summer when powdery mildew usually can be expected to start showing up, you could consider spraying the tree with a fungicide like one of those marked with "H" in the above handbook, e.g., Safer Garden Fungicide.   Fungicides are much more effective when used to prevent or minimize fungal growth than when used as a way to eliminate it.   If you decide to use chemical control, please ensure that you read and follow all label instructions.