Hmm, lendir dalam najis boleh jadi normal especially kalau baby fully breastfed, tapi a few of us tried monitor closely, if the mucus tu dah banyak sangat atau ada darah/bau pelik, memang kena call doctor. Since dia aktif, tak demam, dan minum dengan baik, probably okay lang, tapi jangan risau nak tanya pediatrician during routine check sebab they'll put your mind at ease, better safe with newborns kan!