The green one is an indicator, not a gem socket. So you were already able to operate the cannon, try it again if you interested, and thanks for playing, man.
So, those two yellow cubes are controllers, you can operate the cannon using them. But don`t forget to click one of your thumbsticks first to enable cannon aiming mode and freeze yourself in place for a more persice shot. Click one of them again to enable movement and leave cannon aiming mode.
