Should I consider switching baby formula?

My son is almost 6 months old and he's been on similac sensitive for the last few months. Initially his stools were seedy and had a good consistency, but the last three or four weeks his stools have taken on a dark green and fairly runny consistency. There is also a little bit of stringy flecks to it as well (sorry I know TMI) There are some seedy portions, but it is mostly a very thin consistency of stool; not completely liquid but not the consistency of peanut butter (which I suppose is the average norm). I've called his pediatrician and she said it may be his rice cereal with apples, but I haven't given that to him in over a week. Do you think it's the formula and should I consider switching?