This crab did? How big is it? Looks to be about 1" carapace, maybe bigger, in the photos. I'm not saying it's not the culprit, but most crabs are opportunistic, not obligate predators. Are you sure the sweetlips wasn't already dead/dying?My one ate a baby sweetlips :'(
Do you know, or can you guess, where it came from? My rock all came from the Keys, which has helped me more accurately id a fair amount of critters. Crabs aren't as widely spread as most marine animals, on the individual species level at least.