Take a large plastic bin and place it over the unhealthy moss. If the area is extensive, use as many bins as are needed to cover the unhealthy moss.
Poke holes in the bin, allowing for air circulation. The bin keep more of the moisture in for longer periods of time, allowing the moss to regain its health.
Gently water the moss on a daily basis using a spray bottle on fan mode. Water the moss lightly, using a quarter of the bottle for every cubic yard of moss.
Monitor the moss' health by uncovering the bin and checking to see how green it is. The greener and fuller it is, the healthier.