Moss cannot spread to valid blocks if their top is occupied by non-solid, non-blocks on them.

Vines, Glow Lichens, Torches, Lanters, Carpet, Moss Carpet, Glow Berries, etc. Attempt to bone meal a block of moss next to a stone block with one of those non-blocks on top of the stone. They all block the spread of moss. This does not happen with any other kind of spreading "grass" (grass, mycelium, nylium, etc) in the game.