Tanned hides of big merpeople.
These hides are tanned so that they are tough and resilient, and slits made to articulate at the joints. They are then put on as armor by small merpeople. This way they still look like merpeople but large and unexpressive ones with skin that is not shiny. Armored merpeople seem impervious to harm these large dour merpeople and their enemies suspect they are zombies.
The large live merpeople cannot fit into this kind of armor, but you do not generally hear them complaining too loudly about that to the small merpeople.