Black Spot Remover will Clean this issue, you will need to apply it neat onto the dry surface then agitate with a deck brush and leave overnight If you still have some spots reapply neat concentration onto that specific area again and repeat the process. Once the sandstone is clean and dry apply Stontex Indian Sandstone Sealer and all you will need going forward is the Green To Clean in 12 /18 months when you see any green algae spray the G2C and walk away, the sealer will keep all this staining on the surface making it much simpler to remove and not allowing the stone to get to such a bad state. Also repair the loose jointing mortar with one of our compounds.
Use a pation sealer to further protect go forward.