THe girl obivously didn't understand the facts of 173, he couldn't understnd words or thoughts, and he didn't have a brain, so he just stayed statue until the girl looked away.
Nex: *knife bounces off his natral exoskeleton armor, then picks up knife and adds it to his kitchen, where he butchers pigs and humans* Everybody, I made dinner! ME: Also, it's 173 not 137, silly. Everybody gets that wrong.
The knife clanked off of 173's statue skin. He wasn't specificially made of any Earth stone, it was an inpenetrable unliftable amazingly heavy stone, so the knife just fell to the ground. 173 didn't understand her thoughts nor could it hear it, because 173 didn't techically have a 'brain', so it just stood as statue.