You made too much guac. It happens to the best of us. Press a layer of plastic wrap directly onto the surface of the guac. No air gap.
Then, pour a thin layer of lime juice or water on top of the wrap. The liquid adds pressure and seals the deal. When you’re ready to eat, just pour it off.
Or, do the onion trick in the container. Bury the whole bowl under a blanket of sliced onions. Your guac will stay bright green for a shocking amount of time.