What kind of moss did you use for your wall?
I've tried plastic canvas multiple times, trying to make a christmas moss wall. I think one thing to take into account is that those plastic canvas sheets come with holes of different sizes. The first wall I tried I didn't know this, and bought "7 mesh" black plastic canvas (7 squares per inch) and all of my moss died from not having enough light or water circulation. I then tried five mesh and three mesh before switching to something else, I tied some Xmas moss to a rock in my tank to compare the growth and health against the Xmas moss behind the mesh. The moss on the rick did better for some reason.
Growth was so slow and I got sick of looking at white mesh that I switched to a plastic mesh fabric that I found at a hardware store, as a bag holding a set of clamps. I also switched to Taiwan moss, as it looks similar to Xmas moss but is supposed to be hardier and a fast grower, with smaller fronds that are a better fit for my tiny 2.6 gallon.
I put the moss on a 5 mesh plastic canvas sheet and overlayed with the thin plastic mesh "fabric" I found, sewed it together and have been getting more growth over the last couple weeks than I did in months in my prior experiments
So yeah, find a plastic canvas with larger holes! Or try the mesh bags that oranges or lemons come in, for example.