I've noticed green stuff on the trunks of trees at my home and surrounding neighbors. Leaves (yellow) (large amounts) have beenfalling off a willow and locust tree since the beginning of summer. A cherry tree has yellow ooze as well. All the trees have odd split like areas in the trunk bark. The large locust has about a third of its bare branches looking dead.
Macomb County Michigan
The green stuff on the bark of the tree appears to be lichens. Lichens do no damage to the tree, they just use it as a surface to grow on.
Have you been watering the willow and locust during the dry weeks we have had? If the tree is stressed from lack of water, that may be the cause of the leaf drop. Trees need more water than the lawn during these hot dry spells. The rock around the tree may also be adding to the heat that may be stressing the tree.
Cherry trees may suffer from a problem called gummosis, which is usually the result of stress. Iowa Extension has information at: http://www.ipm.iastate.edu/ipm/hortnews/2011/7-13/gummosis.html
You can always bring a sample into the Macomb County office (call 586-469-6440 for information) or send one to the diagnostic lab on campus for a more accurate diagnosis of problems. Diagnostic lab information can be found at: http://www.pestid.msu.edu/about/