We usually ignore them and don’t even notice them, but they’re one of the most primitive lifeforms on Earth. They’re also good eatin’.
They are lichens, a form of algae with a fungus membrane to support the plant. But they don’t grow in water; they grow on trees and rocks and sometimes on the ground.
Some of the lichens you see in the wild could be hundreds if not thousands of years old. That’s why you shouldn’t pick or scrape lichens arbitrarily. In a survival situation, all bets are off, but if you’re not going to eat it, respect it and leave it alone.
Most lichens are benign, a few are toxic and only a couple have been identified as poisonous. This is not as severe as the mushroom world, where there are numerous varieties that are downright deadly, but you gotta know your lichens.
In fact, only two forms of lichen out of 20,000 varieties are defined as poisonous. One is called Vulpacida pinastri and the other is called Letharia vulpina. Their common names are the powdered sunshine lichen and the wolf lichen.
Both are colored yellow and as a general rule you should avoid lichens that are yellow or orange.