The amount of salt you would have to use to kill the algae will also kill your plants if you don't clean it off the waterfall before turning the water back on. My waterfall rock goes green also from time to time, but I just pull off the dangling string algae and I don't worry about the carpet algae that coats it. It eventually clears.

Some others on here in the past have mentioned Hydrogen Peroxide. I tried it on my waterfall to remove all the string algea and it worked great, didnt kill any plants or fish! Actually I read it can be chemically beneficial as well as asthetically in the summer as it introduces oxygen into the water. I highly reccomend for removing the algea from rocks. I used about a quart for my 2000 gallon pond

koiclay also works and is beneficial for pond and fish.