Several things:
1. Lite is way too close-pull it up at least 6 inches.
2. Pull off the lid and let air flow into the terrarium
for at least an hour or two every day
3. If all you are trying to grow is the moss and the fern
on the wood then I would redo the terrarium altogether
and get rid of the gravel and all the soil. Instead make
up a mix of 50% river or creek sand and sphagnum peat
moss and just put in an inch of this mix into the
terrarium. Make indentations in the mix and press in
the mosses into them. Place the fern wherever.
Then using distilled or r/o water in a sprayer, spray
them heavily but do not flood. Let air for a couple of
hours before placing back on the lid.
4. Keep the terrariumn in the coldest spot you have in the
house if you are using the light. Make sure the temp
inside the terrarium does not go over 65 degrees F.

Note: the moss is dying back because it thinks it is summer
and the high heat is inducing dormancy. Couple that with
wetness around the bottom of the moss and it could induce
fungus infection and subsequent rot. Moss must be kept cool.
The fern generally as an epiphyte will handle heavy watering during fall and spring but only occasional watering in summer but needs good relative humidity 60% to
70%. Check out the weather patterns for the year for your
city. Use the link below to find your state and then find or search for your city to pull up page with weather charts.

Here is a link that might be useful: City info with annual weather charts at bottom of page

Wow, thank you so much! I just moved the light and I think that I might remove the gravel and soil like you suggested. I am sad to hear that 65 degrees is the maximum temperature because my home in georgia is almost always above 75. Will this be a major problem? Is there any way to cool things down other than lowering the house thermostat significantly?

Also, from your post is sounds like, if the moss is going dormant, that there is hope for the future. Is this the case? If i place it in a better environment can I expect a recovery? (and on what time scale...)

Thanks again
Zach