A green fungus-like growth on tree bark can be moss, lichen or algae. Although the growth may look like a fungal disease, it is not. Similarly, the growth of these different substances on tree bark will do no harm to the tree.
However, it can indicate that the tree is not too healthy or that it will die in the next few years. Humid or moist conditions are the main reason for the formation of these growths on tree trunks or leaves. Algae may form after wet weather conditions, while moss and lichen are present on trees typically year round. Lichen is an organism that consists of an algae and fungus that work together in a symbiotic relationship to survive. Lichen growth increases with time, and older trees may have more lichen growth on them. Although these growths will not damage a tree, the growths can be controlled through pruning the tree and boosting the tree's health by giving the right nutrients.