A greenhouse grows moss whether or not you intend it to. The warm, close, dependably damp air that brings seedlings on suits moss at least as well, so in time it films the lower panes, pads the timber staging and works over the rims of the clay pots. Whether that pleases or annoys you depends almost entirely on where it turns up, and once you can read that you can either set it to work or hold it back.
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Why the glasshouse suits it so well
Everything moss wants is handed to it under glass. The air stays humid long after the ground outside has dried, condensation beads the panes and the staging most mornings, and watering keeps every surface damp for hours at a stretch. Light comes in softened and diffused, which moss prefers to hard sun, and the lower reaches of a greenhouse, below the bench and along a north-facing run of glass, sit in steady shade. Most growers water from a butt fed off the roof, so the moisture is soft rainwater rather than limy tap water, exactly what the commoner mosses like. Add still air with little to disturb a settling spore and you have, without meaning to, built close to an ideal nursery for moss.
The green film on the glass is usually algae
Before you blame moss for the grimy green cast on the lower panes, look closer, because that flat film is far more often algae. Moss needs something its anchoring threads can grip, so it takes hold on the timber of the staging, on clay pots, on capillary matting and on gravel trays, not on sheer wet glass. A green slick that wipes off the glass as a smear, showing no little leafy shoots under a hand lens, is a single-celled alga living on the moisture and light alone. The difference is drawn out under moss versus algae, and it matters here for a practical reason: whichever it is, a green coating on the glass steals light from the plants you are trying to raise, so both want cleaning off before the dim months.
Where it does some good
Put to deliberate use, a greenhouse is the easiest place I know to bring moss on. The humidity that sabotages a dry windowsill is precisely what moss fragments and spore sowings need while they establish, and a shaded corner of the bench, or a cold frame stood inside, makes a fine nursery for growing stock on for a terrarium or a kokedama. I keep a seed tray of odds and ends of moss in the shaded end of mine, misted when I remember, and it knits into usable sheets far quicker than anything left outdoors to the weather. The same close air gives a sown moss its best chance, as set out in growing moss from spores. Left alone on clay pots it also lends them that aged, settled look some of us go out of our way to encourage.
Where it becomes a nuisance
The same vigour turns against you in the wrong spot. Moss on the staging and on a solid floor grows genuinely slippery once wet, and a greenhouse floor is no place to lose your footing among glass and clay. A furred-over pane in winter can cut the light a tray of seedlings gets by a surprising margin, drawing them thin and leggy. Moss and liverwort spreading across the compost in a pot or a seed tray is usually telling you the surface stays too wet and the sowing sits too long before germinating, and a dense green mat can genuinely hinder fine seedlings pushing through. Up on the roof, moss in the gutters that feed the water butt blocks the flow and spills it down the walls, a problem shared with any roof and worked through under moss in gutters.
Keeping the balance
There is no need for a spotless greenhouse. The aim is simply to keep moss to the places where it does no harm. Ventilation does most of the work: open the vents and the door on any mild day, and run a fan in a closed house over winter, since moving air dries the surfaces moss and algae depend on. Clean the glass inside and out in autumn so the low winter light gets through unhindered. Sow into fresh compost and let trays dry a touch between waterings rather than keeping them sodden, which starves the surface growth of the constant damp it needs. A scrub of the staging and the floor with a stiff brush sees off the slippery patches, and a gravel or sand topping on the bench drains faster than bare wet timber. Everywhere it causes no trouble, on the clay pots and in the shaded back corner, I leave it be and count it part of the furniture.