Any new rock added recently??? I'm thinking mantis. Maybe not thought since one would think a mantis would've been able to catch the goby by now too. If the crab is the culprit, he is likely only eating someting that died from something else.
On the hermit crab, didja see the shell? is it empty? If not, it may be that it has molted which looks like a dead crab out of the shell. Hermit crabs can also attack snails in search of a new home.
It all may be just normal things that go on in the ocean..... although some just don't trust ANY crab.
"Life may not be the party we hoped for but while we are here we might as well dance!"