Because the Dry Valleys are so dry, normally moss is kind of shriveled and brown. It looks like it might be dead, but it's not! That stage is called "moribund", which basically means "looks dead." As soon as the moss gets wet, it perks right up! In the photo with the green moss, the moss had recently gotten wet from the flow of the stream.
Here's a video through the microscope of some moss. At first, it is shriveled and brown. As soon as I squirt some water onto it, though, the moss opens up and turns green again. I did not change the speed of the video. That's happening in real-time! (Sorry, the video was recorded using my phone through the eye-piece of the microscope, so it's not the greatest quality.)
In fact, a recent article shows how a little warmth, light, and water can revive moss that has been "long dead". Read about the 1500-year-old moss from Signy Island on the Antarctic Peninsula here.