Which movement control is not working? Mouse to control the camera or WASD to walk?
There's two different functions to alter the mouse position, window_mouse_set and display_mouse_set. The manual doesn't make it clear if there's any difference between them (e.g. platform support) but it points out that the game needs to have focus for them to work... I wonder if that's detected differently on Mac perhaps?
To detect movement keys (WASD) when the game doesn't have focus it could be worth trying keyboard_check_direct instead of keyboard_check, it works even when the window doesn't have focus.