Last week the question was: What is this growing on an old piece of wood?
This is a lichen. Lichens are made up of two different organisms: a fungus and an algae (or sometimes a cyanobacteria). The two organisms have a symbiotic relationship where the algae photosynthesizes and provides nutrition to the fungus while the fungus provides protection from the elements for the algae. Since the algal part of the lichen photosynthesizes, lichens do not take anything from from their hosts. In other words, they are completely harmless, though they are frequently blamed for declining plants. Here is a great page from Clemson Extension with more info.
Here is this week’s question: What is wrong with these peanuts?