Lol he really is! I've never seen one so dang small It's maybe not even a inch at the biggest a inch it's tiny. This picture next to the silicone on its tank and the normal size Glass cleaner.
It does look like a smasher mantis shrimp,so its probably your bigger mantis shrimp's baby. Dont ask me how but some animals sometimes can give birth without mateing.
I had one of these come in as a hitchhiker about 27 years ago, from a live rock shipment from Tampa Bay Saltwater. I was 15 and the tank was in my bedroom, I would her snapping at night as the shrimp was hunting.