Starting from macOS Big Sur 11.3.1 Hydrogen has been reported to be difficult to start.

This is due to a new monetization strategy of Apple allowing only applications signed by developers paying top dollars to be executable directly. Since Hydrogen is free/libre and open-source software maintained and developed by a couple of people in their spare time, you, unfortunately, have to do some extra steps to make it run on your Apple device.

Open Hydrogen on macOS Big Sur

Make sure you open the app in the folder with the right mouse button, so you get this message first

Then choose Open also with the right mouse button. You will get a message like this one.

Press "Open" and Hydrogen should start up.