The Place Where Nothing Will Grow gets more attempts at a few kinds of moss (some planted properly, some transplanted with mild care, some frankly just tossed to see what happens) since the ferns are doing fine and the first moss attempts are actually spreading a little.
@moira I don't live in Seattle but that is by far the poorest soil I have ever seen that didn't have like concrete spilled into it or something. If that doesn't work out have you considered doing any but what you have been doing to it?
the top layer isn't actually bad. Down a bit and you get retaining-wall-fill though. And below that actually is concrete.
It's literally a slope fill over concrete. It's not going to get good no matter what. The better answer is hard terracing with gravel (like the rest of the area like it) and planters, but I can't afford that right now.