Greenish stools can also suggest inadequate feeding , starvation stools or may also be a reflection of some pigments in your own food or drug intake.
Also check your food and all intakes generally"
It could be also from the formula especially if rich in iron or if you are on any food supplements drugs or food flavors that can color the stools. Kindly see a Paediatrician for further evaluation
Also check your food and all intakes generally"