Beautiful image Clark.
As posted above the green is just water that's flowing without being aerated into the white froth, pretty much just a thick stream of water over the lip.
Also I second reposting this to be around 700 or 800 pixels tall, even zoomed out as far as Safari will let me I have to scroll up and down the image to take it all in which detracts from such a wonderful photo.
Great job in fantastic light to capture both the falls and the iconic yellow canyon walls. Good composition choice to leave out the sky which can be problematic on overcast days up there. Great work!