There's a lot to like about lichens That greenish stuff that clings to the bark of the tree in your backyard sure looks like a disease, but there's no need to fret. It's merely a grouping of harmless organisms called lichens. A lichen is composed of an alga and a fungus working and living together for mutual benefit.
Some lichens look like paint stains.
The lichen giving a greenish tinge to this bench is composed of an alga and a fungus working together as one harmless organism. Lichens are classed into three groups according to their shape. Crustose lichens are wafer thin and look like a paint stain on a rock or tree. Foliose lichens are leaflike with a wavy or rippled body. The third type of lichen, fructicose, has stringy or antler-shaped growths.