I hate to break it to you, but you either need to remove the coral sand or live with the white stuff on your plants.
My vote is to remove the sand. Your tank has extremely hard water that's not good for a lot of plants and fish. The fact that crushed coral sand works is because its grain size and weight is right. You could keep the waterfall if you get an inert sand with the right size grain. You should look into filtering sands (e.g. pool filter sand). These come in a wide range of grain sizes and are usually (always?) inert.