Will an emarald crab kill/eat smaller fish?

I have three emerald crabs in my 65 - one of them likes to hang out under an acro I have,  in the acro is a very small clown gobie - I never see the crab trying to go after it.

I'm not seeing the threat to a healthy fish.  Perhaps they would attack a dead fish.

 They would have a very difficult time catching a healthy fish with their pinchers, as they are not designed to tear meat but pick macro algae and what not.

It's a crab, mate.  No matter how "safe" they are as a species, you have to bear in mind that (1) almost ALL crabs are opportunistic omnivores in some fashion, and (2) crabs have a mind of their own, and an individual member of a so-called algae eating species can and will go on a carnivorous rampage if the mood strikes. 

Ever been pinched by one of those algae-harvesting claws?  They still pack a little punch in their pinch. 

Its been my experience that the smaller emerald crabs are generally harmless, but once they get big - and especially if there isn't a lot of algae to scavenge - they will start hunting down whatever they can get.  Larger fish are off the menu (unless dead I suppose), but I have seen them threaten smaller fish before.  I'd suggest that you make sure you're choosing a larger watchman goby, and best be sure that it is a very healthy specimen.

+ one with bang guy. Also if there isn't enough algae to satisfy them you could all so give them a treat once a week of any type of food you feed your fish with or a piece of a nori sheet of there own.