Mould can be collected through use of Shears, but can only be farmed on Rotten Wood. Using it as a fuel in a Composter allows the composting of four items a piece.
It can be combined with four Supernatural Velocity and four Lapis Lazuli pieces to make Cultivated Mould. This variety of Mould is light blue in colour and grows at insane speeds, spreading across a 5x5 farming space of Rotten Wood in under three minutes! The only downside is that it only composts two items per piece instead of four, but the growth rate more than makes up for that.