but I am not sure what they look like
They look like a praying mantis
I also heard purple pseudos will prey on them, by the way, are purple pseudos good reef fish, aggressive wise and so forth.
I've never heard of this fish eating mantis shrimp...the other way around maybe... I have heard/read that most pseudochromis will eat bristle worms, but not mantis shrimp.
If you want to remove unwanted critters from your rock, precautions are in order. One of the best methods I've heard of is to make a batch of SW
using carbonated water (seltzer) rather than RO
. Prior to putting the rock in your tank...hold the rock in the carbonated SW
for a minute or so. The fizzing/bubbling will drive them out of the rock, you should have a net ready cause desirable critters will also flee the rock. The rinse the rock in some regular SW
and place it in your tank