Check out the first picture. Is it chewing gum? Is it seagull poop? No, it's Aspicilia. This is a lichen that likes to grow on damp bricks, which our courtyard provides in abundance during the winter. Every Fall, I watch the colonies spread slowly and multiply, until the courtyard is sometimes home to hundreds of them.
We know that many lichens like to grow on trees, while others prefer rocks. But here we have one that seems to have adapted happily to suburban living, and doesn't even mind being walked on occasionally. The second picture is a close-up of a colony, showing the little nubs of fungal tissue containing algal cells.