User:Steed has suggested moon bacteria or moon germ.
Talk about it here.
Spider germs were gigantic bacteria-like creatures that appeared on the surface of the Moon, which was in reality the eggshell for a gigantic draconian creature. Their main food supply came from feeding off the creature that was growing inside the moon, similar to bacteria from Earth. They took the appearance of spiders, and attacked humans when they made sudden movements. (TV: Kill the Moon)