What is causing these white spots on our maple tree?
Looks like the crustose lichen, Phlyctis argena, often mistaken for a disease. They are not harming your tree. From Hortsense: Lichens are primitive often found growing on the bark of older trees and shrubs. They are typically grayish, greenish, or orange in color and may be flat or branched. Mosses and lichens are not parasitic and will not harm the trees. They are often associated with shaded or declining trees, but do not cause tree decline. Regular pruning usually limits development of mosses and lichens.