Maple Tree Disease
This is powdery mildew on the maple tree. This fungus starts much earlier in the season and develops into this white growth. There are forms of this fungus that get on flowers and vegetables as well as trees though they do not spread between the different families of plants. It is not considered a serious problem but can reduce photosynthesis as the whitish growth covers portions of the leaf. Since leaves are surviving and functioning this late into the season it is having a minimal impact on the tree. The disease becomes more noticeable in late summer as conditions become more favorable for it to develop. Cool night temperatures and warm days with high humidity helps the fungus to spread. It is considered cosmetic and no treatments are called for this late into the season. Why it could be on one tree and not another is often related to the specific localized conditions such as shade which would increase the humidity around this plant. Please see the following website for more information on powdery mildew.