Several trees in my yard have flat, gray or green scale-like growth on the bark; is this something bad?
The growths observed on the trees are lichens.
A lichen is actually composed of two different organisms, an alga and a fungus, which grow together for the mutual benefit of one another. These crusty or leaf-like organisms may be a variety of colors, including brown, gray, green, yellow, and white. Lichens form on a variety of surfaces, such as rocks, soil, and fallen logs, as well as on the bark of living trees. When lichens are noticed on trees, they are sometimes mistakenly thought to be a disease organism. Lichens frequently appear on the trunks or branches of trees that are in poor or declining condition. Soil compaction, poor drainage, excessive drainage, and injury are just a few possible causes for reduced growth vigor.
While their presence may indicate there is a problem, they are not in themselves harmful.