how to get rid of pleurococcus from garden slabs?

we jet washed ours when they got like that. they came up a treat

It is often a question of time with this one. Jet washing takes more time than a patio cleaner. With this in mind, anything that is growing near must be protected, the glaze on your patio if not natural stone can be damaged. At this time of year I use Armillatox which will kill Pleurococcus and after 48 hours will be safe to walk on and no damage to pointing.