Ball of Moss is a part of a Tinkers' Construct mod.
When combined with a tool or weapon on the Tool Station, that item will slowly regenerate durability over time, making it an almost essential modification for tools. Standing in direct sunlight will increase the rate of regeneration. Does not stack, uses one slot. Moss can be applied to an item multiple times (each ball of moss using one modifier slot) to increase the rate of regeneration.