A moss terrarium should be near-effortless, so when it goes wrong it is usually one of a handful of causes. Match the symptom and fix the cause.
Browning or crisping
Almost always too dry, too hot, or too much direct sun. A closed jar in a sunny window cooks in minutes. Move it to bright, indirect light, mist with rainwater (hard tap water also browns moss over time), and make sure a closed lid is actually holding humidity.
White fuzzy mould
Too wet, too still, and usually too much decaying organic matter. Remove the affected pieces, take the lid off for a few days to dry and air it out, cut back on watering, and remove dead leaves and debris that feed the mould. A little airflow now and then prevents it.
Glass streaming with water
A light mist on the glass is healthy; water running down it means it is too wet. Leave the lid off until the inside is merely damp, then close it again. Persistent heavy condensation breeds mould.
Pale, leggy, stretched growth
Too little light. Moss does not want strong light, but in deep gloom it weakens and stretches and algae take over. Move it somewhere brighter but out of direct sun, or add a low grow light.
Fungus gnats
Little black flies usually mean the substrate is too wet and rich. Let it dry back, improve drainage, and they generally fade. Bought-in moss laden with soil is a common way to import them; a rinse first helps. See the terrarium guide for getting the setup right from the start.