Moss

From DwarfFortressWiki

Though moss is in-game, it is unknown as to what causes it. Moss potentially appears after a long amount of time, such as after reclaims. Current known objects subject to moss are stone block constructions exposed to sunlight. Current known cases of moss occured in the following biomes: Temperate Conifer Forest (Warm Woodlands), Freezing Woodlands with Magma Vent/Aquifer.

Moss turns the tile it is on green. It is unknown whether dwarves will clean up moss, or whether traffic will destroy it like grass.