Why do you have a green bowel movement?

Related to food. Likely related to food items you've eaten. Or sometimes to non-food items: ingesting foods that have been colored with food coloring can change the stool appearance transiently. My brother once ate blue chalk. Yes- you guessed it right.
Green food usually. Dark green leafy veggies can cause green stool., iron supplements, algae rapid transit through the gut can cause bile to get into colon (often causes diarrhea). Lots of GI causes: infection-salmonella, giardia, celiac sprue, malabsorption, ibd, gastroenteritis, and more. Ongoing, associated with other symptoms, diarrheal stools, pain, fever, etc- see your doc.