Tree moss is a type of LICHEN, which is a composite organism consisting of a fungus and an alga living together in a symbiotic relationship. Lichens are found in many different habitats, including on trees, rocks, and soil. Tree moss is a type of LICHEN that grows on the bark of trees, providing them with nutrients and helping to protect them from the elements. It is also an important food source for many animals, such as birds and insects.