Different kinds of resins in mortar contains organic nutrients which
enables algae to assail the mortar.
The mortars coarse surface gives good hold to both damp and dirt, which contains further nutrients for the spores of the algae. The affection can therefore establish itself faster than normal.
The spores establish affection that recurs –as long as there remain spores and rests of algae on the surface. Algae have to be sanitized.