One or two episodes of green stool can sometimes just be from something you ate, so it is not always a cause for panic. That said, since it came with a loose, watery consistency, it is worth keeping a close eye on how you feel over the next day or so. If it happens again, or you start getting stomach cramps, fever, or feel generally unwell, I would go see a doctor just to rule out anything that needs treatment. Better to get it checked and have peace of mind than to second-guess yourself at home.