Many oaks have a delicate covering of lace green living material, hanging down in curtains.
Oaks often have real mosses, often growing as a carpet on the upper surfaces of large branches that are well shaded. Ramalina needs sunlight and water to grow. It can sit in the hot, dry summers of California, or in a museum drawer, for very long periodds of time, and appear dead. But as soon is it is wet and in the light, it springs back to life. We don't know why it grows more abundantly on some trees and not others, given an equal amount of open branches without leaves in the winter on otherwise similar trees.