Powdery Mildew

This looks like powdery mildew, a fungal disease that starts out as small circular areas with spore development primarily on the upper leaf surface. Be sure to collect and dispose of all the fallen leaves instead of composting, as spores can overwinter on dead leaves.

In our region, potted Japanese maples are more likely to be affected than those planted in the ground. Luckily, the disease is more unattractive than problematic in the long term (especially in the stage your photo shows). It’s usually at its worst in late summer (or now!) due to persistent dry conditions. Prevention focuses on a healthy root system – so make sure the plant isn’t under-potted (meaning the roots are crowded around the edge of the root-ball and overheat in hot weather). Poor drainage could also contribute to susceptibility. Neem oil is one method of control available at your local garden center, but at this stage in the season, you may choose to wait this out. (Very young foliage – typically earlier in the season – can damage the growing tips, but this is not evident in your photo.)