I have a 12 year old irregular shape flagstone patio that I have previously powerwashed to clean. However, powerwashing is a very slow laborious process. Also, the powerwashing is not too kind to the grout even if you are very careful. So, the flagstone grout lines are darkened and dirty in some spots, there are darkened streak lines on some of the flagstone and a light green algae stain on some of the flagstones. I have been told various things ranging from 1) use liquid bleach mixed one part bleach to one part water, 2) use TSP (trisodium phosphate?) to 3) use muriatic acid and cut it. I use muriatic acid in my swimming pool but it is nasty and if it is too strong it will quickly damage stone, concrete, etc. I would rather not use muriatic acid if I can avoid it.
I also need to clean the concrete deck around my swimming pool which has some very dark green/black algae forming on it as well.