Fey moss itself is useless. It’s a black, scummy substance. If left unchecked it will cover almost any surface with a thick, tar-like substance.
Sunlight almost instantly destroys fey moss, causing it to burst into flame. Unadulterated fey moss is also extremely flammable.
But, like the plankton of the ocean, fey moss is the bedrock of the underdark’s ecosystem. Ecosystems on the surface all ultimately draw their energy from the powerful rays of the sun – plants capture that energy; herbivores eat the plants; and carnivores eat the herbivores. But in the underdark the ecosystem ultimately derives its energy from fey moss (which, in turn, draws it directly from the magical ley lines).
Animals in the underdark either eat the fey moss directly or they eat the wide variety of fungal species which have adapted themselves to parasitically grow upon the fey moss. Civilized species establish vast fungal gardens to feed their populace.
A similar growth exists in Gamma World, absorbing background radiation and enabling larger creatures to metabolize said energy.