FT, David (DRH) turns the waterfall off, lets the algae dry up a bit and then pours Hydrogen Peroxide 3% over the waterfall rocks. It causes the algae to bubble up and then it can be blasted off the rocks, with a sharp stream of water from the garden hose.
Hydrogen Peroxide 3%, will not harm the fish or plants in your pond and is a great deal cheaper to buy than those other products.

I use a small stiff brush to clean off my waterfall rocks, with the water still running. The fish gobble up the bits as they fall into the pond. I have noticed, that as the weather warms up, the algae contains those little "Bloodworms" (Midge and Black-fly larvae etc) that the fish love to eat.
"Horton"