Moss in paving divides people sharply. To some it is a slippery nuisance; to others it is the soft green seam that makes old stone look settled and right. Both views are fair, each in its own place.
Why it grows there
Paving joints and gravel collect grit, organic dust and moisture, and in shade they stay damp long enough for moss to take hold, especially where drainage is poor and the surface rarely dries out between showers. Fresh mortar between slabs is often slightly alkaline, which keeps moss off for a while, but as it weathers and a film of grime builds up the surface turns more welcoming and moss moves in. Because moss takes up water across its whole surface rather than through roots, a joint that stays damp for a few hours after rain is all the reservoir it needs.
What actually grows in the cracks
The mosses of paving are mostly tough, low, drought-hardy sorts that cope with baking sun one day and a downpour the next. They manage it through poikilohydry, letting their water content rise and fall with the weather: they dry to a crisp in a dry spell and revive within minutes of a wetting rather than dying. That is why a path can look bare and brown in August and green over within a day of autumn rain. Honest naming often stops at the genus here, since the plants are small and the features that separate species need a hand lens, but the growth habit is easy to read. Tight upright cushions favour the drier gaps, while flatter creeping mats take the joints that stay wet longest.
When to clear it
On steps and well-used paths, moss is a genuine slip hazard when wet and worth keeping down. A stiff brush and hot water shift most of it, and a patio cleaner does the rest. The lasting fix is to dry the surface out: improve the drainage and cut back overhanging growth so the stone gets light and air between downpours. Avoid pressure-washing soft or old stone, which pits and erodes the surface and leaves it rougher, so moss returns faster than before. A dry, well-lit, free-draining path simply gives moss less to work with, which beats scrubbing the same slabs every year.
When to encourage it
On a quiet, shaded path, a terrace edge or between stepping stones, moss in the joints is a feature people work hard to fake. To encourage it, leave the joints alone, keep the area shaded and damp, and brush a little moss slurry into the gaps as in the slurry method. The Japanese stepping-stone look, with stone set into a soft moss field, is exactly this, done on purpose. Rainwater suits it better than hard tap water, which over time leaves a chalky film many mosses dislike; there is more on that in water for moss.
Living with the balance
Most gardens want it both ways: moss gone from the busy, sunny route to the door, and moss left to soften the still corners where nobody hurries. That line is easy to hold once you see what drives it. Shade and damp make moss; light, air and free drainage unmake it. Change those conditions and the moss follows, with far less effort than repeated scrubbing. Leaving some moss where it does no harm also does the garden good, since a mossy joint holds moisture and shelters small life; see moss and wildlife. Treated as something to steer rather than something to defeat, moss in paving stops being a battle and becomes part of how the place looks after itself.