I made a separate page devoted to internet “research” on this subject. It may be that a blender slurry of buttermilk and moss is an urban legend as Mossin’ Annie says. I tried to use a food processor and never got to to a slurry state. It ended up as clumps of moss with a buttermilk/liquid mix. I spread it on some Durock and left it behind the house in a moist and shaded area. But I have little hope of it working. I will try the method of the third video on Can You Grow Moss.
What is a proper substrate for a vertical wall? Possibly a fiberglass window screen smeared with a clay and dirt mixture – attached to a stronger wood or wire structure.
Or clay and mortar for something more durable.