When I tried opening it there was a message that said I couldn't open it because "The creator couldn't be verified" :(
Hello there, thank you for wanting to play Belong~ Rest assured that the game is safe to play, and I suggest this guide on how to bypass it ^^ hope it helps!
Edit: checked with a macbook friend and it may be Mac just being really cautious because of an unknown developer. Ctrl+right click the app can bypass it too. Updated the guide link above!