Could you please clarify what a terrarium with good drainage means? (I.e. Any way you could post a photo of the whole terrarium just for reference?) The photo shows a fern sitting on a moved clump of soil. To me at first, the plant appears to be inundated with water and not enough light to balance the environment in the terrarium. Ferns are known as shade lovers but they do still need enough light.

That's selaginella, which is really a moss (clubmoss), not a fern. But it enjoys pretty much the same conditions as a fern - bright but indirect light, consistently moist soils but not boggy and relatively high humidity.

Since it is in a terrarium, which is going to generate its own humidity naturally, I wonder if misting would be overkill. Maybe move it to a slightly brighter/warmer area so it can dry out. At some point, I think you may have to remove some of the dying foliage, but maybe there will be new growth in spring if it recovers. These are such cute plants! Good luck.

Thanks for the correction on the moss....I saw it, and knew it, but just went with fern anyway. Lol But I would still try for more light and let it dry a little bit (no misting)