Lichens are bluish-green to gray paper-thin plants that grow on the outside bark of the tree. Their growth becomes very dense and upright on some plants, while on others they are flat on the bark of the tree.
Lichens can also be found on rocks and other inanimate objects indicating that they are receiving no nutrients from the host tree, as is the case with mistletoe. They are a combination of algae and fungi growing together in what is known as a symbiotic relationship. The trees are involved only in that they are the object on which the lichen is growing. That means they are almost always harmless to the tree.