Forklore: Yum scum
When they were being whimsical, people used to claim the moon is made of green cheese. They were not referring to a cheese that's green in color, of course; green cheese is simply the opposite of ripe or aged cheese. It is a pale lunar white, like cottage cheese; ripened cheese tends to be tawny, like Swiss or Gouda.
But there are cheeses with green veins in them, like blue cheeses. And in 16th century Mexico, there was a green "cheese" that was dairy-free. It was the edible pond scum, Spirulina geitleri.
The Aztecs, who called spirulina tecuilatl, harvested it from Lake Texcoco using nets or even shovels. The places in the lake where the scum grew were owned, like fields or orchards, and their owners often grew rich by renting out the right to harvest from them.
The algae would be spread out in the sun until it was dry enough to be shaped into small cakes, which were put in the sun to dry further. Today we think of spirulina as a chlorophyll-rich health food; to the Aztecs and the Spaniards, it was a flavoring. The Aztecs put it on toasted corn or tortillas.
When fresh and green, the flavor reminded the Spaniards of cheese (although "with a certain taste of mud"), which is why they called it queso. With age, tecuilatl turned blackish and developed a strong flavor and could only be used in small quantities.
-- Charles Perry