How to store Flame Moss out of the tank?

Keep it in a water filled jar in light.

That's what I do.

Sunlight in a window or light next to the tank?

I plan on keeping it out of the tank for many months

light near a tank I would think. worst comes to worst, could you not keep it in a ball somewhere in the corner of the tank? I've done that before too. I stuck mine in the corner of the tank, but the UGF stack and where I keep my heater. I think I still have some there.

I have a couple different mosses in a 1 quart tupperware sitting on a windowsill. They are doing fine.

If you are worried about nutrients, put 1/2" to 1" of yard soil in the jar, swirl around, let it settle for a couple minutes, pour off the cloudy water/clay mix, refill with fresh water, and your good to go. (You don't have to pour off the clay/cloudy water, only if you are impatient to have all the soil settle and have clear water.) No soil cap/gravel needed if you don't plan on messing with this often and stirring things up.

-Dave

Heck, I am turning some into terrarium. riccia, monosolenium tenerum and various mosses.... So dave's method would work a charm.