Crab killer?

OH MY! Well its hard to tell from the video but it looks like either a mithrax crab or possibly a rock crab. If its a rock crab get it out ASAP. If its a mithrax crab they are reef safe (with caution) my mithrax had a tendency to eat small hermits and snails and was picking on my conch. Somebody else will probably have a better idea for the ID of that crab

I agree, it is a rock crab, so called because they come in on ocean collected LR as tiny guys and grow larger in our tanks, sometimes catching and eating fish 10 times their size. True story.

The easy way to catch rock crabs is with a pint sized glass jar or drinking glass. Put a small piece of raw shrimp in the jar and place it at about a 45 degree angle or a little more of an angle against the rock near where the rock crab lives. Do this just before lights out. During the night it will crawl into the jar after the meat but the slick glass sides will keep it from crawling out. This often works the first night but may take several days before the crab builds up the courage to investigate the tasty morsel.