Spagnum is probably not the best stuff to plant your plants, unless you are using meat eating plants for two reasons:

1.your plants will sink into the moss and might kill your hermit crabs.
2.it is hard to landscape because of the last reason.

Also, your hermit crabs, because they are herbavoris, will eat your plants.

Thanks for the advice. I put in coconut husk substrate.

Hermit crabs need a humid dry environment. They need dry sand to dig in--at least a couple inches--so they can molt regularly. The air inside the tank needs to be at the very least 70% humidiy; 80% is better. The temperature needs to stay around 75F. That's how my crabs like it, anyway.

They are omnivores and may eat the plants--but they don't eat much, so it probably won't hurt. I had ivy in a hermit crab tank and they didn't eat it. I had a fern and they ate it in a week. But at that point, I wasn't providing the food they like best, so that could be prevented, maybe.

I've added the link I used to set up my hermit crab tank.

I'm here because I'm wanting to make my tank, which I set up for the crabs, more like a terrarium. So I'm coming from the opposite perspective. 8^)

Here is a link that might be useful: Hermit crab care

Every time I've put a plant into my crab's tank, even one in a pot! they've dug up. Not that they ate it... But they seem to have a mission to dig up any plant I put in there.

And hermit crabs are nocturnal and like low light, how would you provide enough light to grow plants?

I also have sand in my hermit crab's tank and I let it dry out every so often to prevent mold and stuff.