When cooking mung beans you should soak them for at least 4 hours prior to cooking the mung beans because it helps the mung beans cook faster and much better especially if you're cooking the mung beans in a pot on the stove and not in a slow cooker.
Soaking the mung beans for 4 hours minimum is recommended but you can soak the mung beans in water overnight and then cook them the next morning.
If you do not soak the mung beans for at least 4 hours prior to cooking the mung beans they will just take a bit longer to cook.
You don't have to soak mung beans before cooking them but it's recommended to soak the mung beans or any other beans before cooking them so that they cook better and faster.
You can also just put the mung beans in the slow cooker and cook the mung beans overnight or when you're away from the house such as at work, shopping etc.