Green Mossy Wall

Moss chambers scattered through the underground and cavern layers are already papered with this backing behind their glowing growth. Hammering the natural sheets out returns nothing at all, so exploring is not a way to stock up. The item form only exists because you can build it yourself, which means what you carry home from a chamber is the moss, not the wall.

The naturally placed backing counts as unsafe, so the room in front of it keeps spawning enemies like any open cave and cannot hold an NPC. Sheets you put up yourself are treated as normal player walls, and the space goes quiet immediately. That gap matters if you want to live inside a moss chamber: clear the original backing, replace it tile for tile with your own copies, and the room looks identical while the spawns stop. Each moss color has its own matching wall, so a mixed chamber can keep its natural color scheme.