I've got crabs! (What are they?)

[Blizz003]

Hi! I'm new here and to aquariums in general, but I'm a chemist so I'm still fairly overconfident...
I bought a piece of live rock form my new tank because that seems to be the thing to do...
It came with a mushroom that I fear probably won't make it...
There were also several serpent stars that ended up being food for the crabs. One is definitely an emerald and he's far bolder than the other guy that I'm not sure of. The only time I've seen the second crab is when he came out to try and grab scraps from the emerald who was enjoying the serpent.
Now, if I want to get some clowns in a few weeks and maybe eventually do some corals, what should I do with these crabs? Will they need to come out? I've attached a the best pic I could get of the unknown crab... If anyone could ID him that would be super.
Thanks!

[hillegom]

Not a very good pic.
What kind of pincers does the unknown crab have? If sharp, probably a meat eater, if like the emerald they are dull, then maybe he will eat algae.
But all crabs are opportunists. eventually they all are bad. Depends on how hungry they are.
You could catch him in a trap. put some food into a long glass, lay it with a slight angle on a rock. Hopefully the crab goes in and its too slippery to get out. Then you could take him down to the LFS to ID

[Blizz003]

Thanks! That's what I was thinking... He looks very similar to the emerald, just not very green... And he has bright red eyes... I'll see if I can't get a better picture...

[hillegom]

Not all emerald crabs are green. JL was selling some emeralds that were dark grey to black

[Blizz003]

I'm fairly certain it's some kind of mythrax. . It looks just like the emerald but with different colours. I recently added two cardinals and 1 bicolour angel. The cardinals are doing well but I think the angel got ick from the sress of moving tanks. He was fine at the LFS and when I first put him in my tank he seemed super happy, exploring everywhere and inspecting the snails and crabs... But then two nights later he was on his side up against the glass breathing really heavy. He also has some orange spots on the yellow part of his head and along his gills which is what makes me think ick... He'll swim up next to the crabs until they pick at his skin, poor guy. I don't have a hospital tank and I'm not sure what to do. Will neon gobies help or do I need to medicate the poor fellow? Before you ask, the water parameters are all good and he still eats mysis. Ahhh poor thing he just started rubbing against the rocks I feel so bad...

[Myka]

Looks to be Mithrax to me too.