I think it's better to grab the rock, then use a turkey baster over a bucket with cold tap water (unsalted). This way, you don't shock the entire rock, just the section with the mantis. It's worked for me.
The best way to remove hitchhikers is to buy some carbonated water, adjust the salinity to match your tank, then dunk the LR
in it for about a minute, rinse in another bucket of tank water, and replace in the tank. That way you can pick through and keep which hitchhikers you want. The fresh water method pretty much offs most of the hitchhikers.