being that the crabs are intact it sounds like a molt.... but what do I know?

If those crabs are actually dead, then it most certainly was NOT a mantis. I've seen mantis attack crabs and what is leftover is not intact in any way.

Hmmmm, thanks guys, seems so weird for their to consistently be clicking noises coming from the tank and then this and not to be a mantis but yeah, some clues suggest there is one and others suggest there is not, so I guess Im back at square one lol

Well, you guys solved my mystery. I bought an emerald crab about a month a go, and found it dead (or so I thought) last week. A couple days ago I see ANOTHER emerald crab truckin' around in the tank! I only bought one, so I thought it might be a hitchhiker.
When Crabs and shimp molt, they tend to hide for a few days after as their shell is still soft and they are vulnerable to attack. Wait a few more days and keep your eyes open. If they molted, they'll turn up soon.

Kelly

I had trouble with emeralds as well, one did great for a while so tried adding a second which did not last then the first one dissapeared so I gave up on them.

Unbelievable...
 

So one of the little buggers turned up today, bigger and more bad ass than ever. Im happy.

I simply cannot believe that what I found was molt. It looked EXACTLY like a full crab. Sweeet!