DECK WASHING THINGS TO KNOW BEFORE YOU BUY

How can it kill algae although not vegetation? I have a scrap marble and mortar patio (concrete slab beneath). Will this clean this equally as very well? I utilised an acrylic deck stain which didn’t delay perfectly. A lot of the stain has pealed. I don’t want to use the same stain. What does one advocate to go over the top of the present acry