This is powdery mildew, a family of fungi which infects many species of plants.  Powdery mildew is generally not a problem on Japanese maples unless they have bad conditions that encourage it, i.e. bad air circulation or lack of sunlight so that leaf surfaces stay wet for prolonged periods of time. 

Even a Japanese maple that normally doesn't have this problem may get it if the weather is ideal for powdery mildew.  If the disease showed up late this year, the good news is that the leaves will soon fall and the appearance of the disease was probably weather-related.  Next year, assess its planting site: maybe over the years it has gotten more shady or thick or crowded by other plants that are cutting down the air circulation.  In that case, some pruning may be called for.
Here is a link to info on powdery mildew on our website:   http://extension.umd.edu/learn/powdery-mildew

ECN