Green poop?

My LO is formula fed she went from one brand to another. but she has been on the new brand for a month now. As of last week her poop turned green for a few days. My concern is could it be the formula changing the color or should i believe my MIL gave my 2 month old a herb she keeps saying she wanted to give her and i said no. I started working the past two week, this being my second week. I dont want to go say anything to her about it if it could be the new formula or maybe its a change because of my baby growing. Mommas can someone shed a light for me.