Protecting paving slabs from algae?

Well a professional concrete/cement man the patio would be flat (foundation as deep enough so no leaning = puddling) The steps would have a slight downward grade so that you can't tell but water runs off/down, no more that 3%. Try A Chalk (like you use in bathroom/tubs.) for grout. Water proof sealer like thompsons will help but it too degrades over time