Here's a little tip on that from a marimo care page:
"Marimo turning brown may be a sign that it’s time to gently clean it. If this doesn’t fix it, try carefully picking off the brown (dead) parts and adding a bit of salt to the water to stimulate the Marimo’s growth. If the bottom of the Marimo has turned brown because it didn’t receive light for longer periods of time, be sure to start rolling the Marimo around a bit more often to prevent parts of it from dying off again."
And when it says "salt" it means aquarium salt. Of course, do that in a separate container that has just the marimo in it. Don't worry about it not being in a heated environment. Some sites actually suggest that a cooler temperature can fight off the browning too since marimo do well in cold water as well.
It also seems that too much light can also cause browning, so you may want to do the above steps and then roll it so that a different part will be getting the light.
