Moonlight Policy means additional employment beyond regular working hours by an employee without notifying his regular employer.
Since the side job was mostly at night time or on weekends, it was referred to as moonlighting.
The term gained popularity when workers in America started looking for jobs other than their regular employment of 9 to 5 for additional income.
Recently, food delivery start-up 'Swiggy' has allowed its employees to take additional employment after their regular employment period in the company.