Why my poop is green?

Fast transit time: can do this ( IBD, celiac, food allergies ). A fast transit time (shorter than 10 hours) means food is passing through your system too quickly. Not enough time the bacterias in the large bowel to breakdown the bile and change the color from green to brown. The ideal bowel transit time is anywhere from 12 to 24 hours. Also some green color vegetables , iron pills , color in the foods can do this.

Diet can affect it.: The composition of your diet can at times affect the color and consistency of your stool.

No issues: Stool color is determined by the foods you ingest and the bacterial flora in your gut. It can vary from yellow to green to brown. I would be concerned only if it is black, red or has no color at all

Depends: What are you eating. Many vegetables like mustard greens, spinach may impart a greenish color to stool. It is not an abnormality and you can safely ignore it.