It's mostly a type of lichen. It would have more of an effect on more common types of roofing materials - very little on slate. It is a soft-vain slate quarried mostly from Bangor and Pen Argyl in Northampton County, PA. It is quite porous as compared to slate quarried in most other areas (all slate is porous). The U-shaped discoloration on the surface and the powdering underneath is from many decades of moisture absorption and to a lesser degree, leaching of gypsum to the surface. Please remove that "classification" system from your vocabulary. That is a grading system invented by folks that made it up to sell an ASTM standard. It's completely inappropriate to use to determine the age or remaining life of a slate roof. What's the age of the house, the slope of the roof, the headlap, the type of nails and percentage of slates needing immediate replacement?