Wa'alaikumassalam! Hmm, poop color changes boleh jadi normal especially kalau baby fully breastfed, but since it's different from the usual kuning color he's been having, better to check with your pediatrician just to be sure lah, a few of my mum friends had similar worries and the doc put their minds at ease quickly je. Is baby acting normal otherwise, feeding well, and having wet diapers okay?