"Look at all the deer poop," James commented while we were outside the other day. No, I said, that's not poop. It's some kind of slime. That's all I knew. Which wasn't much.
So I took some photos and went online. According to Michigan State University Extension, our slime is "Nostoc sp., a genus of cyanobacterium formerly classified as blue-green algae.... Following a period of rain, it may appear suddenly in lawns, pastures, paved surfaces, roofs or stones...."
For more reading on this interesting stuff, read this article in Jim Conrad's Naturalist Newsletter. I'm not going to worry about it. Our green slime is just part of nature!