The Creeping Moss is an item added by Hunting Dimension.
It is linked to a specific biome, and has the same color as Grass in that biome. Sneaking and right clicking with it will change the held stack's biome to the biome the player is currently standing on. Right clicking the ground in the Hunting Dimension will consume a Creeping Moss to change the biome of the chunk to the one the Moss held. This can only be done if the current chunk has a different biome than the Creeping Moss.