Ah, you've cornered me into a confession. The left side is actually an interesting tale. When I first planted this it was all hairgrass belem in the back. I later decided it needed something more so added the stems. Later, at one point I thought I would try some riccia on stones around the hardscape. It wasn't long before I decided I didn't like it, so pulled them out. The riccia had other plans and invaded the hairgrass. I didn't keep up with removing it and it soon was using the hairgrass as a scaffold to grow thick without floating. I began to just trim and shape it. The substrate is well below the slope.
But, a tip for getting substrate to slope high is to use substrate supports and/or DSM to get it to stay. Plastic credit cards or gift cards can work or other plastic pieces. The Green Machine sells their TGM substrate supports to prevent substrate slippage.