I used to take care of my good friend's baby girl when she had to go back to work. My LO was a year old and I never remembered her having green poops or mucus in stool. So when the baby started to have them I told the mom. The baby was gassy and would feed small amounts at a time, (fussy at nipple). I had to always have her in carrier to help with gas.
After I got pregnant again I stop taking care of her the mom started to care for her. I think that's when she noticed it was a problem. Well it turned out to be a dairy and egg allergy. Oh the baby was constantly congested. So it does sound like an allergy to me.