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 are correct, in the right 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. The mortar between slabs is often slightly alkaline at first, but as it weathers and grime builds up, moss moves in.
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; a patio cleaner does the rest. The lasting fix is to dry the surface out: improve drainage and cut back overhanging growth so it gets light and air between downpours. Avoid pressure-washing soft or old stone, which it pits and erodes.
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, stone set into a soft moss field, is exactly this, deliberately.