Monitor it. If it happens another 2-3 times, then consult a medic. It could be nothing, or something that may have been consumed in his food, that normally isn't in it, I.E. a mistake somewhere along the line. If so, you would expect the problem to attenuate over time, when the green discolouration became less and less, or just stopped.
If the problem appeared persist after another 2-3 times, to the severity it did the first time, it might then be worthwhile consulting a medic.
I'm presuming it's not causing him any other issues, like Stomach pains, pain when releasing stools, some kind of anal rash, or other possible related things, like Vomiting, Headaches etc.