Lichens can grow on almost any kind of surface, including the bark of trees both live and dead. They extract nothing from their substrate, I.e., they don't harm the trees on which they grow. They come in a variety of textures and colors, but none of them are really "green" when they are dry, though some are yellowish-green and a few become greenish when wet. They often grow with mosses, on trees, rocks, and on the ground.

I see lichen growing on the bark of living trees all the time. In fact, I see it more often then I see moss. Most of the lichen I see around here is a kind of blue-green.

Lichens are fascinating organisms-mutualistic combinations of a fungus and and alga, and sometimes a fungus and a blue-green alga (which is actually a type of bacteria) and sometimes all three, working together! One wonders how that all ever got going, but I'm sure glad it did, as these organisms are both beautiful and interesting. BTW, lichens are indicative of good air quality.

+oM