Shade does more than block sun
A slab in full sun dries within a few hours of a rain. The same slab under a tree canopy, on the north side of a house, or behind a fence can stay damp for a day or two longer, and that extra time is what moss needs to get started. Shade also usually means less air movement across the surface, which slows drying even further on a day it is not raining at all.
Texture holds moisture differently
A broom finish, the rough texture most driveways and walkways get for traction, has more surface area and more tiny pockets than a smooth trowel finish, and that texture holds a thin film of moisture a little longer after a rain. It is not the reason moss appears on its own, but on a slab that already sits in shade, the rougher finish gives spores a slightly easier foothold than a smooth one would.
A damp, shaded slab versus one that will not dry at all
Most moss is nothing more than shade and moisture doing what they do, and it comes back after cleaning because the shade and the moisture are still there, not because anything is wrong with the concrete. What is worth a second look is a slab that never seems to dry between rains even in a spot that gets some sun, or one where moss keeps returning thick and fast within weeks of being cleaned off. That pattern points at water lingering on the surface longer than ordinary shade would explain, which is a different question worth asking on its own.
Getting it off and keeping it from coming back fast
Moss lifts with a stiff brush and a cleaner made for it, or with careful pressure washing if the surface can take it. How hard to run a pressure washer on your particular slab is its own question, covered on our page on pressure washing concrete. Once it is off, thinning a tree limb or two, cutting back what is blocking sun, or clearing gutters and downspouts so they are not adding extra water to the shaded side goes further than any one-time cleaning does.
A sealer, especially a penetrating sealer that soaks into the surface rather than sitting on top of it, will not stop moss on its own, since the moisture is coming from the air and the rain, not soaking up from below. It can make a future cleanup a little easier by leaving the surface less porous, but shade and drainage still decide most of how fast moss returns.