It was my first time to Yellowstone and I was surrounded by so much beauty! This is Yellowstone Falls. The green coloring at the top of the falls is apparently due to a rounded notch in the rock where the water is flowing.
Edit: Just heard back from Yellowstone about the green coloring in the water at the top of the falls. This is what they had to say:
"There is a “crack” in the edge of the rock where the water falls over the edge. So the green stripe is just where the water is deeper than the rest of the falls and it takes longer for it to aerate and turn white like the area around it."