Artificial grass is sold as the low-maintenance answer to a mossy lawn, so it comes as an unwelcome surprise when the fake turf itself starts greening over. It can and does, wherever it lies damp and shaded. The consolation is that nothing in a plastic lawn is alive for the moss to invade, so the green is only sitting on the surface, and it lifts far more readily than moss rooted into soil ever would.
Why a plastic lawn turns green
The moss is not really growing on the plastic; it is growing on everything that collects among it. Pollen, blown dust, leaf litter and the fine organic grime of any garden work down into the pile and pack into the sand or rubber infill, and in a shaded, poorly draining spot that layer stays damp for weeks on end. Moss and algae ask for little more: a moist, sheltered surface with a scrap of organic matter to sit on. So the patches that green over are the predictable ones, the north-facing strip, the ground beneath a tree or against a close-boarded fence, the low corner where water lingers after rain. The fibres feed nothing themselves; the muck caught between them feeds plenty.
Getting it off
Begin dry. A stiff broom worked briskly across the pile lifts most loose moss and the debris underneath, and sweeping against the lie of the fibres before smoothing them back reaches the base where the growth clings. For green that has bedded in, a bucket of warm water and a firm brush shifts the bulk, and a purpose-made artificial-grass cleaner handles the rest. Two tempting shortcuts are worth refusing. The iron-sulphate mosskillers meant for real lawns do nothing useful here, since there is no grass for them to green, and the iron rusts orange stains into a pale pile. Running a pressure washer at full power is the other trap, because the jet blasts infill out of place and can fray the fibres or lift the backing; if you reach for one at all, keep it wide, weak and well back.
Rinse, brush, dry
Whatever cleaner you use, rinse it through so nothing is left behind to hold damp, then brush the pile upright and let the surface dry out completely. A lawn that drains and dries is one moss struggles to recolonise, so the drying counts for as much as the scrubbing. In a shaded patch that keeps greening up, an occasional wash with a dilute artificial-grass or patio sanitiser, the benzalkonium-chloride sort rather than bleach, holds algae and moss down without staining the fibres or harming the borders alongside.
Keeping it from coming back
Prevention is mostly housekeeping. Brush the lawn over every few weeks so debris never builds into the damp mat moss needs, and be quick to clear fallen leaves in autumn. Trim back anything overhanging that throws the surface into permanent shade, since a few hours of sun and moving air each day achieve more than any cleaner. Where a whole area stays sodden, the fault usually lies below: a compacted or badly laid sub-base traps water that no amount of surface brushing will fix, and it may need lifting and relaying on a proper free-draining bed.
Not the same fight as a real lawn
It helps to understand why the familiar moss advice does not carry over. On a living lawn you beat moss chiefly by helping the grass win, by feeding it, aerating the soil and opening up the shade until the turf grows thick enough to crowd the moss out, as the moss and lawns page sets out. Artificial grass hands you no such ally, because there is no plant to strengthen, so the whole task comes down to denying moss the damp and the debris it feeds on. In that respect a plastic lawn behaves more like a patio than a meadow, and the general approach to clearing hard, shaded surfaces in how to get rid of moss fits it far better than any lawn-care routine.