Could you please help me with identifying this new creature. My original idea was that it is Cymo melanodactylus.
But it's relatively big, my guess is around 4cm. It's hidden inside the rocks during the day.
Any ideas, hints?
It looks like a gorilla crab. I'd remove it, but id you have a sump, you could keep it there.
Several things on crabs typically indicate a problem. Large heavy claws, or the crab looks hairy, or the back leg on each side ends in a paddle, rather than a point are typical indicators of a problem species. If you see any of these, it's usually best to remove the crab.
I removed a few 'problem' crabs with a small glass jar wedged against the LR with some prawn/shrimp at the bottom. Crab goes in, can't get out - unless it's one of the swimming crabs as DaveK mentioned, i.e. has paddles for back legs. That sucker will just eat and swim out again lol.
Don't leave it in for long though, you don't want ammonia creeping up. TBH, can't remember exactly what I did now, was over two years ago. Good luck.