B.C.
My first thought was teething (but shots and allergies are also considerations). I suggest homeopathics and the Hyland's Kid's Kit. It's about $30 and I think you can get it at Sprout's and Whole Foods.
In the kit, Chamomilla is the remedy recommended for teething and "green-ish slimy stools" is the 'guiding symptom'.
I like Hyland's for the baby's as it disolves in thier mouths. Stick the remedy between their cheek and gum. It's sweet and they like it. There are 6 remedies and a bump and bruise ointment in the kit. I found it invaluable!
B.