This symptom is caused by powdery mildew, a very common and usually minor fungal disease. It shows up on a wide range of plants this time of year due to environmental conditions of warm days, cooler nights, and sufficient humidity. You can refer to the linked page for more information, but fungicide treatment is both not needed nor effective at this point. You can rake-up fallen leaves and dispose of them if you prefer (trash, not compost), as a way to reduce spore abundance for next year, but it's not a foolproof way to ensure no repeat infections in a future year, since spores can be windblown from further away. Fortunately, powdery mildew is generally just an eyesore and it doesn't cause much impact on long-term plant health, at least with Japanese Maples. (That's the type of maple pictured, a red-leaf variety of Japanese Maple, Acer palmatum. Our native Red Maple, Acer rubrum, has leaves shaped differently, and their foliage isn't red during the growing season, only autumn.)

Miri