A simple pendulum oscillates at a period given by the expression
P = 2 pi sqrt(L/g),
where L is the length of the pendulum and g is the acceleration of gravity. The mass of the pendulum bob does not enter. On the Moon the acceleration of gravity is less than it is on the surface of the Earth, so the period on the Moon will be greater than it would be at Earth’s surface. This results in fewer “ticks” of the clock in a given period of time. Therefore a clock on the Moon will run slower than an identical one on Earth. The Moon clock will lose time compared to the Earth clock.