Dead or Moulting?

[Dreadhorn]

So I was just walking by our hermit crab tank, when I noticed my one crab is out of its shell, and laying quite still. I picked it up to check it it was dead. The long part of the body seemed fat, and healthy, and its body is paler than my other crabs, so now after looking at google, I'm thinking CRAP is it molting, or dying?
Granted, this is the first time in my adult life that I've had hermit crabs. A lady at work said her son received them as a door prize at the beach (dont get me started). She didnt want them, and had them in a 5x3 inch tank.
I said yes, so back mid-september, I brought them home, got them a suitable living space with proper water (which they had none of).
They seemed to be well as far as I could tell.
I'm never sure how much they are eating because their food seems to puff up and mold after only a couple of days (if anyone has suggestions on what i should do about that issue, it could help).
I had already moved the crab, so I took it, and its shell, and put them in the old small tank, and put it under a couch in the dark.
I'm not sure if I'm doing the right thing though, and I also dont want my other crab to injure it if it IS moulting. So what can/should I do for it? I've been trying to go by google up until this point on care, and not sure where to go from here.
Thanks!

[Wai]

You should change whatever food you put in there at least once every one to two days. The crab sounds like it may have died. Moulting hermit crabs occasionally do abandon their shells, but they will usually crawl back in. A dead crab will start emitting a fishy odour after two or more days and begin decomposing.

[Dreadhorn]

No change this morning, and part of the body is looking a little shriveled, so I think its done for. I had thought maybe moulting because I forgot to mention before but the ends of its legs were also all missing when I got it. Poor thing. Well I still have one to try to take good care of. Thanks for the food info.

[Dreadhorn]

Also, how often should I change the water in the dish?

[Wai]

Ideally every day, but every two days if you can't manage that.