Here are some reasons that cause the moss in terrariums die.
1. Waterlog
Moss starts to turn black where there is waterlogging. Most times black spread from bottom up. Sometimes the black starts from the sides.
Look at the moss below. On the surface, this piece of moss still looks green. But when you break it apart, its roots turns black.
2. Direct sunlight
In summer, the light is intense. Moss will turn yellow where it is exposed to sunlight. This is very easy to recognize. The piece of moss on my hand died due to accidental exposure to direct sunlight. Look carefully at the bottom, it's still green.
3. High temperature
This is another reason why moss dies. All the moss turns yellow or black for high temprature. In summer, if the temperature is too high, exceeding 27°C / 80°F, moss is likely to die. Please note that in a closed terrarium, without air conditioning, the temperature inside the glass will always be several degrees higher than the outside. Please maintain ventilation and increase watering frequency to cool them down.
I do not list low temprature here because in the room temprature is always above 0°C / 32°F and there is no need to worry about this problem.
4. Mold
I Mold will cause moss die. I list it here because it is easy to solve. Spraying some bactericide for plants and the moss will recover in 2 weeks.