Have they had a cold or anything as sometimes being unwell or teething (anything with mucus or saliva) can make it this way
My LO (EBF) had Green mucusy poos with smell and is diagnosed with Cow’s Milk Protein Allergy. Look out for other symptoms as well which might be related to (CMPA).
Baby girl had poos like that when she was poorly, she was really congested and only latching for a couple of minutes at a time. My HV said they need to feed for around 5 minutes before they get to the fatty milk, so I also put it down to her, not feeding for long enough. She still has a green poo every now and then, but i think as long as they're gaining weight nicely it's nothing to worry about x