Here’s my ignorance: I thought lichens were a thing.
It turns out they’re a composite organism, that is, composed of two living things—a fungus and a fungus’ friend, according to the Wiki-P, “usually either a green alga…or cyanobacterium….” My sense is that they grow outward, so that bigger blobs are older than smaller ones (this is a larger older blob, although there’s nothing to give you scale—you had to be there…).
More Wiki-P wisdom: “Lichens are named based on the fungal component, which plays the primary role in determining the lichen’s form.”