It could be vitamins, if you are taking any with green coating. I had it happen to me after taking green coated vitamins.
Also, I don't know if it's just me this happened to before, but one time after eating cake that had blue frosting on the edges of the cake, I had bowel movements that were blue for a few days. It could be something you are taking (as in vitamins) or something you are eating. Try to think of what it could be.
I wish I could remember the name of a bacteria that one of my kids had several years ago but it turned there poop bright green!!! It really freaked my out. I'm not sure if it effects adults but it took a few weeks with meds to go away. I THINK it starts with a "D". I will get back in touch with you later when I remember the name.
Also, such things as Koolaid or drinks that contain blue food coloring will produce the flourescent color when mixed with bile. Not to worry, but I think I'd lay off that kind of stuff long term.
You are eating something or taking a supplement that has green coloring in it, like chlorophyll, for example.