I have number of Maple trees, each around 4 years old. Lately their leaves have become a lot smaller with white mildew spreading out from the veins. They also have a green/yellow lichen on the branches which seems to be taking over the bark. I don't think the lichen is an issue but I am concerned that with the stunted growth it will completely cover the tree. I have 2 images if that would help you identify the problem.
Lichens and moss are generally not an issue on tree bark.
The white mold spots you discuss are likely a fungus of some kind. The fact that you have a number of trees effected can be of concern.
I would recommend a professional arborist inspect your trees for a proper identification and treatment coarse.