Thanks, everyone.
Wayne, the lighter areas that look yellow(er) aren't dying. That area just doesn't receive as much water from the waterfall. It's also the area the squirrels often try to dig in to hide the peanuts a neighbour feeds them.
Just a word of warning with moss on rocks on waterfalls...it will divert a fair bit of water, eventually. There is a lot of my water that drips through the moss, to the sides. The moss is growing out, and in between the rocks. As I don't have the rocks "set" in foam, just stacked, I may have to cut the moss back in a few years, on the backside. I'm good for now, though, as the liner is right to the back edge of the rocks, and rolled. It does mean I have to top up often, though, with such a small pond. Moss is thirsty. LOL