Here's what I would do.
I would freshwater dip every piece of rock for a few seconds. No longer than 10, then get the rock back in. Check the water carefully, anything could have popped out that you would want to save. Second, after you get the rock back into the water, watch it carefully, I've had a mantis come out only after I put the rock back into the tank. If it is a pistol, it'll probably come out, same with mantis. The bottle trick won't work on a pistol, they wait for food to come to them, not vice versa. Unless they have a goby with them, but thats still a hit or miss thing. Good luck with your decision!
Richard