Moss and terrarium plants in general can benefit from humid, temperate conditions. If you fear your terrarium is getting too hot, you have a couple of options for cooling it down.
My first recommendation is to attempt to cool your terrarium by keeping it in a cooler spot (away from direct sunlight or sunbeams). Perhaps near a curtained window? I searched on the web for some low tech tips that also might help you and found some good ones at josh frogs.com. The suggestion about placing an icepack or a frozen water bottle on top of or inside the terrarium sounds like something that could be easily attempted with minimal investment or risk of over-cooling!
Have you tried another method for cooling a terrarium?