I struggle with keeping Neps and Nep. cuttings alive, especially from Fall->Winter. I tossed 7 of them that had started growing and then went down hill. I do not have an artificial light and a true terrarium, but have tried creating my own version of one by purchasing a storage container with a lid. I partially fill the conatiner with water and place the plants inside, propping them up so they aren't immersed. They were kept at a SW facing window sill and the cover was slightly open. It failed miserably.
I was turning it over in my mind, when Summer arrived, to use hanging baskets, suspended from a tree, dangling into a fish tank filled with water but not immersed. I am trying to balance sunlight, humidity, and air circulation.
In the meantime, a friend suggested a method that is far more simple: Get two plastic deli containers. One is taller than the other and its purpose would be to be filled partially with water and to support the shallow container, that would rest on top. The shallow container would have holes poked at the bottom and contain soil media and plant (uncovered). The water level would be just below the shallow container and it would be exposed to the air. In theory, water would percolate up through the holes, keeping the media and plant moist, but not immersed, so as to prevent rot. Being uncovered would prevent mold and maximize light, whether inside (for Fall & Winter) or out.