HELP, fish missing and coral dying...

[Whatigot]

I have noticed my mated true perculas have been acting agressively towards my royal gramma and I am assuming that they killed my clown goby as he is suddenly gone without a trace.
I noticed last week a little black crab in the tank that must have come in on some new live rock from ocean aquatics I bought 2 or 3 weeks ago.
Over the last 3 days, my leather finger coral has been torn apart pretty good, not dead yet, but I am sure another attack will prove fatal.
I hope some of you more experienced reefers can help me, I want to know if the crab I saw could have not only eaten the coral, but also if it might have killed and eaten my clown goby...he loved chilling on that finger coral.
I also want to know how I can catch the little bastard if at all, I have only seen him once at 4 in the morning and I was way to inebriated to start messing around my tank at the time, so I stupidly left it.

If not on the little black crab with skinny claws, I am blaming the death of my clown goby on the mated percula that came together with it in the aquapod I started reefing with.
please help, I don't want to lose anymore coral or fish.
K.