Algae. It is fed by sludge and sunlight. My pond has done this for years. I think I may have it on the run now; it looks better for this time of year than it has in 10 years. I am using a fountain for oxygenation and I currently have 2 aerators running to activate the bacteria on the bottom. Copper sulfate will kill the algae temporarily but then it falls to the bottom and creates excess nutrients so it becomes a vicious circle. Last weekend I rigged a "cowcatcher" on the front of a 12' v-bottom and circled the pond pushing the algae out the overflow. Even before that I only have about 10% coverage as opposed to the normal 60%.