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. Fructicose lichens, the least common, can be found in the subarctic where reindeer nibble on them for food. Lichens come in a wide variety of colors, including bright yellow, gray, pale white, blue-green, black, and reddish. In dense shade, they are slow growers, but in sunlight, they grow fast.