What Causes Greenish Stools Pain Around The Navel After Appendectomy?

My 7 year old son had an emergency op to have his appendix removed 10 days ago. The appendix was gangrenous and moderately bad although it had not ruptured. He has over the last couple of days had green slimy faeces - the stools are softish although not diarrhea. Is this normal? He is otherwise recovering well although he was complaining a little yesterday of tummy ache around the belly button and he does look rather pale. Thank you