Glad to hear you're enjoying the game!
To exit the backroom, you have to interact with everything in there. That's four cabinets,a desk, and a piece of paper. (if I recall correctly) When you've interacted with everything, Clam Man should walk to a corner of the room and trigger a dialogue. Have you tried exiting and reloading the scene? Or maybe there's a tiny object you missed? Hopefully this solves your issue, let me know if it doesn't!