I found a video that really looks like the mantis I have, same color and maybe a bit bigger.
http://www.youtube.com/watch?v=begwA3NiCDg
I have read that only smashers live in the liverock and the spearer only live in the sand and rarely make it as hitchicker because of that, so I probably have a smasher. I guess they are much less dangerous for fish then?