Just water can work pretty good. For stains on concrete I have had good success using a cheap powered laundry detergent mixed with water to make a paste. I will scrub this on any stains then keep it wet by spraying it once in a while or cover it with a plastic tarp for a few hours. For green (alge or vegetation) stains mix a little bleach in. I have had stains from oak leaves that never went away... It must be something unique to that particular leaf.
It's the oak leaves that I could not get off with bleach or hydrogen peroxide (oxy clean). Someone told me it was tannic acid in oak leaves that cause the stain on cement. I do not know if that is true but I can tell you the oak leaf stains in driveway were there atleast two years and might still be... I moved so I don't really know.
I use some pool chlorine and water mix but that isn't too good for vegetation and it seems to work fairly well but there is still a ghost image. I may do some googling for a better remedy; if I find anything I'll post it.
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