Greenish stools happens when baby sometimes has loose stools.If the baby is gaining adequate weight,passing urine normally and not vomiting it should not be a concern.

Greenish colour stool can be physiological. Its is considered normal if baby is accepting feeds well and growing well. In your case if baby is taking feeds well no need to worry but if you notice baby is passing increased frequency of loose watery stools and dull looking you should look for dehydration that is passage of urine normally or not.