Does my 4 month old have diarrhea?

i havnt changed anything in his diet... just formula.... and ceral mixed in just before bed like the doc. said 2 months ago... he's 4 and a half months old. he pooped a litle this morning and it was like yellow mustered. now he pooped again and its like a pastel green or like my father says like a spinich green... its been like this for a couple of day, but nothing else has been going on with him... is this normal for him?