Kill it with fire!
Anyone who’s been on the internet for a couple of years now will probably know what this creature is. All you need to know is that the OP found it inside of the mouth of a fish that they were cleaning. We hate to remind you guys of what sort of horrors there are in the underwater world, but we gotta.
This is a Cymothoa exigua, also known as a tongue-eating louse. They latch onto the tongues of fish and steal whatever food goes into the fish’s mouth. It’s pretty disgusting, so this restaurant definitely needs to have a talk with its supplier.