No, it's murkier water. The microorganisms cause the light to scatter, giving the water a duller appearance. Similar to how scuffed up glass will appear duller

No, it's murkier water. The microorganisms cause the light to scatter, giving the water a duller appearance. Similar to how scuffed up glass will appear duller than polished glass. The microorganisms can also have a strong color, the most obvious example being algae. I imagine most of us have seen a bucket of water left in the sun turn green after a couple of days.