Theres a thread in the heath section named "the scoop on POOP" that Nancy wrote that states:
"Over indulging in a new food can cause green poop. All new foods and treats should be given in a very small amount to start. A few kibbles, or a mouthful or two is enough to start. If there is no reaction, the following day add a bit more. Never introduce more than one food or treat at a time."
Actually after reading, I found this sentance that states exactly what you're asking about
"Stress can cause all kinds of stomach upsets with our hedgehogs. This will show as anything from a slight green tinge to otherwise normal looking poop, green soft yet formed, green mush, diarrhea, mucous of various shades or neon green jello."
-Also from that thread that nancy wrote
yes a new cage can cause it. Keep an eye out for his newer Bowel movements and see if they appear normal. If they are okay, then it was prolly jsut stress. If he continues to produce green pop, then he may be sick and may need vet attention.