According to Hank Green and the good folks at SciShow, one of the most commonly googled questions on Earth is "why is my poop green?" Why, indeed?
The short answer: Your food went through you too fast. That, or you recently ate a bunch of leafy, green vegetables. The slightly longer explanation, which Green delivers here, involves a primer on bilirubin, a substance produced by your liver.
For info on why your stool is black, red, or yellow, greasy, and foul-smelling, see here.