Dark green poo can happen with formula fed babies, especially if baby's digesting the iron in the formula well, but since it's been going on consistently I'd suggest bringing baby to the PD just to rule out anything and get their peace of mind. They can check baby over quickly and let you know if it's just the formula or if there's something else going on, usually it's nothing to worry about but better to ask than wonder.
https://sg.theasianparent.com/baby-poop-colour