Bleach is the reach-for-it answer to green staining because it's cheap and it visibly lightens whatever it's poured on. That's also its limitation — bleaching something isn't the same as killing it, and on outdoor stone the difference shows up again within a season.
A biocide treatment is a different category of product, formulated specifically to kill moss, algae and lichen at the root rather than to bleach their colour out.
What bleach does and doesn't do
Household or patio bleach oxidises the pigment in algae and moss, which is why the green colour visibly fades. It doesn't reliably kill the organism's root structure sitting inside the pores of the stone, so regrowth from the same colony is common once conditions turn damp again.
Bleach can also lighten the stone itself, particularly on natural sandstone or limestone, leaving patchy pale areas that don't match the surrounding surface — a cosmetic problem that outlasts the algae it was meant to remove.
What a proper biocide is designed to do
- Formulated to target the specific organisms — moss, algae, lichen — rather than to bleach colour generally.
- Applied with a dwell time so it penetrates into the pores and joints where the roots actually sit.
- Continues working after application as the organism dies back over the following days.
- Designed with run-off and plant safety considered, rather than being a generic household product used off-label.
The regrowth pattern that gives bleach away
A bleached surface often looks clean within hours, then starts showing faint green again within a few weeks in the same spots it was treated — because the same root survived. A biocide-treated surface takes longer to look fully clean but the die-back is genuine, so the same colony doesn't simply reappear from where it was left.
If a driveway or patio needs repeated bleaching every month or two to stay presentable, that's usually a sign the treatment never reached the root cause in the first place.
Cost and staining considerations
Bleach is inexpensive and available anywhere, which is its appeal, but the patchy lightening it can leave on natural stone is difficult to reverse. A biocide costs more per treatment but is formulated not to bleach the stone itself, so the surface keeps its natural colour once the growth has died back.