People clean moss off a family grave for sound reasons: a name has disappeared under green, or the stone simply looks uncared for. It can be done gently, and it can be wrecked in an afternoon. Old stone is softer than it looks, and the tools that shift moss quickest are the ones that carry the carved face off along with it. A little care, and an honest look at whether to clean at all, saves a good deal of regret.
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Check before you touch it
Two questions come before any water. Is the stone yours to work on, and is it protected in some way? A memorial in a churchyard belongs to the family that raised it, yet the ground beneath it is the church's, and many parishes would rather you spoke to the incumbent before cleaning. Anything that alters an older monument may need formal consent through the church's own courts. Commonwealth war graves are looked after by the CWGC and should be reported to them rather than tackled yourself. Listed and historic memorials carry legal protection, and a well-meant scrub can count as damage to them. When the stone is old, notable, or not plainly your own, the safe course is to ask first.
The stone decides what you can do
What is safe depends almost entirely on the rock. Marble and limestone are soft and, being forms of calcium carbonate, dissolve in anything acidic, so they etch and mark with ease and must never meet an acid cleaner. Sandstone is porous and often friable, shedding its surface in flakes when worked too hard; on a crumbling sandstone face a moss cushion sometimes holds more together than it spoils. Granite and slate are hard and far more tolerant, which is why polished granite is the one material most people can clean with real confidence. Weathered Victorian stone of any kind is more fragile than a modern offcut. Where you cannot tell, treat the stone as the softest thing it might be.
The gentle method
Nearly every stone worth cleaning can be cleaned with plain water, a little time and a soft brush, and nothing stronger. Wet the surface well and let it soak, which swells the moss and loosens its hold. Work from the top downwards, so dirty runoff passes over stone you have not yet reached rather than over stone you have finished. Use a soft or medium brush of natural bristle, never wire, and let it do light work instead of scouring. A wooden or firm plastic spatula lifts thicker moss out of the lettering without scratching, where a metal blade would score the stone for good. Rinse as you go. Expect to come back over two or three sessions rather than stripping it bare in one, and take a faint green shadow left in the grain as a fair price for an intact surface.
What to keep well away from it
The fast methods are the ruinous ones. A pressure washer clears moss in seconds and peels the weathered skin off the stone with it, opening the surface so it drinks in water and greens back sooner and deeper than before. Wire brushes and abrasive pads leave permanent scratches and snag on the edges of the carving. Household bleach leaves salts in the stone that go on drawing damp and can bloom white across the face for years, and it scorches the grass and planting round the base. Acidic products, which take in many patio and brick cleaners and anything sold for descaling, bite straight into marble and limestone. The internet's fondness for bleach, vinegar and cola on graves is best quietly ignored; none of it belongs on stone you would like to outlast you.
Biocides, used sparingly
For growth that is stubborn or comes racing back, there is a gentler chemical route than any of those. Conservators reach for slow-acting biocidal washes, usually built on quaternary ammonium compounds, brushed on well diluted and left to work over weeks as the rain carries them through the stone. The best known goes by the name D2, and it is widely used on memorials and war graves. These are far kinder than bleach and leave no staining salt behind, but they remain chemicals: follow the dilution to the letter, keep it off the surrounding plants, and on any stone of value take advice before you begin. Patience does the greater part; the wash only sees it through.
When to leave the green
Cleaning is not always the kinder act. On soft old stone a coat of moss can shelter the surface as much as hide it, and stripping it lays tired stone open to frost and driving rain. Lichens, slower again and often a hundred years in the growing, are worth sparing wherever the inscription still reads through them. Where a cushion has merely softened a stone without swallowing its meaning, letting it stay is frequently the wiser call. The mosses that make old burial grounds so rich, and the case for valuing rather than scouring them, run through the mosses of the churchyard, and the wider argument against harsh cleaning of any weathered surface is in removing moss.