Formula-fed babies can have a range of poop colours from yellow to tan to greenish, and it can shift depending on the formula itself, so there may not be cause for alarm if baby is active and feeding well. That said, since this is your first time seeing this colour and you are not sure, it is worth bringing a photo or even a sample to your PD just to get a proper check, because they can tell you straightaway whether it is something to watch or just a formula thing.