Minecraft wiki entry for Processor_list, associated text:  mossiness: Must be between 0.0 or 1.0. Each block affected by this processor generates a number between 0.0 and 1.0; if this value is greater than the threshold value, it will be replaced. Slabs and walls use mossiness as their threshold value, but stairs and full blocks always use 0.5, and obsidian always uses 0.15.