Emerald crab dying or molting?

[oKrazyii]

So I got 2 emerald crabs roughly 4 days ago and they’ve been doing completely fine, but today I walked into my fish room to see this one on its back twitching and holding its one leg? It’s next to a rock so I know it’s not stuck and the other emerald crab and all other fish and inverts are doing perfectly fine, all my Params are fine so I see no reason it would be dying but it’s now been doing this for roughly 4-5 plus however long it was like this before I walked in this morning, I’ve attached a YouTube video to show what’s happening I’m just a little confused and concerned, really hoping it’s just molting. I fed fish flakes right before taking the video to see if they’d move at all because they normally get very excited and start grabbing at the sky when I feed (I normally feed a little heavy so the snails, emerald crabs, pithos crab and peppermint shrimp have a fair bit of food which they cleanup pretty fast) and as you’ll notice there’s no reaction from them even when the food floats directly into their face.

 

[Elira_6301]

I've read that some people have had experience with their emerald crab molting upside down. Just keep an eye on him. He may disappear while his exoskelton hardens if he is indeed molting.