My baby had "green ng ng" as well but he is on mixed-feeding. The nurses told me those are the minerals (e.g. potassium etc...) that my baby did not digest. Not sure whether that is the same case as your baby.
Joyce
祝善謙氣宇軒昂、俊逸不凡!
善良忠實氣宇軒,
謙遜有禮俊逸顯。
Thanks to Joycelyn for such a nice poem!