A moss terrarium should be close to effortless, so when it goes wrong the cause is usually one of a small handful of things. Nearly every problem traces back to humidity, light or airflow being slightly off, and once you learn to read the glass the fix is quick. Match the symptom to its cause and you will rarely lose a jar.
Read the jar first
Before changing anything, look at the whole setup for a moment. Where does it sit, how much sun reaches it through the day, is the lid on or off, and when did you last water it? Most terrarium trouble is a mismatch between the moss and its position rather than anything wrong with the moss itself, so the answer is often to move the jar or adjust the lid rather than to intervene inside it. The commonest single mistake is a closed jar in direct sun, which turns into an oven within minutes.
Browning or crisping
Almost always too dry, too hot, or too much direct sun, and frequently all three at once. A closed jar in a sunny window heats fast and scorches the moss against the glass. Move it to bright but indirect light, mist with rainwater rather than hard tap water, which browns moss over time as the lime builds up, and check that a closed lid is genuinely holding humidity rather than sitting loose. Browned moss is often only dormant, not dead, so give it a few weeks of steady damp before writing it off.
White fuzzy mould
Too wet, too still, and usually too much decaying organic matter feeding it. Remove the affected pieces at once so it does not spread, take the lid off for a few days to dry and air the inside, cut back on watering, and clear away dead leaves and debris. A little airflow every few days prevents mould taking hold in the first place, which is why cracking the lid is worth doing as a habit rather than a rescue.
Glass streaming with water
A light mist of condensation on the glass is a healthy sign that the jar is holding its own moisture. Water actually running down the inside means it is too wet. Leave the lid off until the inside is merely damp rather than dripping, then close it again. Persistent heavy condensation is the classic run-up to mould, so treat it as an early warning rather than waiting for the fuzz to appear.
Pale, leggy, stretched growth
Too little light. Moss does not want strong light, but in deep gloom it weakens, stretches and loses colour, and algae move in to take advantage. Shift it somewhere brighter but still out of direct sun, or add a low grow light on a timer. The change is gradual, so give new light a few weeks to show.
Fungus gnats
Little black flies drifting up when you lift the lid usually mean the substrate is too wet and too rich. Let it dry back, improve the drainage layer, and they generally fade. Bought-in or wild moss carried home with plenty of soil is a common way to import them, along with their larvae, so a rinse and a look-over before planting saves trouble later.
When nothing seems wrong but it still fails
Sometimes a terrarium simply sulks for a few weeks after building while the moss adjusts from open air to the still, humid world under glass. A little browning or melt at the edges early on is normal and often recovers. Resist the urge to keep opening, misting and fiddling, since constant intervention causes more losses than patience does. Get the position and the humidity right, then mostly leave it alone. See the terrarium guide for getting the setup right from the start.