Mold blocks can be found on the Stalactite layer underground and require at least a Stone Mining Cell or an even stronger Power Cell to be mined.
Mold can be processed into 6 Globs of Goo each in a Processor. No crafting recipe has to be unlocked for this.
These natural blocks are used in a few crafting recipes like for Siltstone Wall or Purification Bombs (together with blocks of Stalactite, Trog Horns and Gunpowder).
Blocks of Mold can placed and made into artificial Biomes where Warmworms and Mirus will occasionally spawn in complete darkness, but also Obsidian Treasure Chests now and then.