My kids and I could be wrong but we suspect a Mantis shrimp in our 28 gallon of killing our Manadarin Goby. After I moved the aquarium to Houston, I noticed that he was in the usual rock that we had always seen him in (now on top of the rest of rocks). So, I pulled the rock out, set it in a bucket and proceeded to fill it with fresh water until he evacuated the rock and then returning the rock to the aquarium asap.

Your tactics will have to vary a bit and unfortunately, I think you will probably have to remove all the rock from the tank. He's probably got a burrow under your rocks that he'll retreat to once you try to net him.

Here's what I would do:

1. buy or obtain 2 20 gallon storage buckets and a 5 gallon bucket. These make great temporary holding facilities for water changes, transports, or rock rearrangements.
2. fill the buckets with saltwater that matches the params of your tank. Personally, I would include a small heater and a power head for your live rock buckets.
3. remove the rock until you can trap your pistol shrimp with a net.
4. put the pistol shrimp in the 5 gallon buckets
5. put all the rocks back in your aquarium.