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!