Depends how you're growing it, I suppose. I'm probably not the best qualified to asnwer this, but here's my two cents anyway: The moss needs to be kept moist, if it dries out then it dies. You can give the moss the moisture that it needs a number of ways, putting a cover on would be a suitable way, but you need to watch your temperatures. If you need to worry about high temperatures for example, if you grow using sunlight, then it might be easier just to add a bit more water to compensate for the water that will be lost. My observation is keep the temperature from getting too high (covered container outside in full sun = too high) and keep it wet. You can accomplish these any number of ways. All in all, I'd say it be better to just give it more water instead of worrying about humidity, if it has sufficient water, then it shouldn't burn and you should be alright.