Why is your bonsai moss dying?
The causes of the bonsai moss withering are fertilizers for bonsai tree, too much watering, too much or not enough sunlight, not suited to the climate, excrements of birds and infestation by insects/fungi/mold.
Fertilizer for bonsai tree
Moss only requires water and sunlight to thrive. Fertilizers for bonsai trees could be too strong for the moss (like overfeeding them) and can cause it to turn brown or die.
Moss does not have developed roots as plants do to absorb nutrients, or even water, from the soil.
Depending on the species, they either use their absorbent surfaces to draw in nutrients and water from the water that flows over the outside of the moss. They can actually absorb nutrients from the water-conducting shoots.
Or, some of the mosses are able to pull nutrients/water up the external surface of the rhizoids (roots-like organs) and to the stem by capillary action.
Either way, moss does not need strong nutrients source like fertilizers to live, which can be harmful to it.
Too much watering
Watering adequately for the bonsai tree may be too much for the bonsai moss if the soil is completely covered by the moss.
The bonsai moss can repel water and prevent water from getting through to the soil when it covers the soil entirely. When this happens, we have to water more than the amount needed for the moss to give water to the tree.
It is true that the moss lives in moist places but overwatering, soaking in the water, is not what they can tolerate.
Too much sunlight or not enough sunlight
The degree of sunlight the moss gets can be the cause of your bonsai moss dying.
Moss needs sunlight as much as plants do for photosynthesis from which it produces energy. Having said that, the amount of sunlight can be too much for the bonsai moss depending on the moss species and where the bonsai pots are placed. For example, shade-loving moss like undulate atrichum moss needs less sunlight.
On the other hand, photophilic moss like silver moss needs sunlight. If your bonsai tree has thick foliage, it can block the sunlight that the moss needs.
Not suited to the climate
Bonsai moss species that do not fit the climate of the region where you live can be the cause of your bonsai moss dying.
If you live in California whose climate is warm and dry, moss species that prefer high humidity and less sunlight might not thrive well. If you live in Florida where it is hot and humid, mosses that prefer a rather dry climate might not grow healthy.
Excrement of birds
Bird droppings contain uric acid and are very acidic. It is good for plants as fertilizer but if bird excrements touch moss, they burn the moss and the acid absorbed by the soil will slowly damage the moss.
Infestation by insects, fungi and mold can weaken the moss and lead it to die.
Some insects, including spider mites, and worms eat the green part of the moss as food source. They also feed in rhizoid, a root-like structure that holds moss to the soil. This can weaken your bonsai moss and turn it brown.
Bonsai moss can also be infected by fungi or mold, especially if the bonsai tree is kept inside. Fungi and mold love places with high humidity and no airflow.