The Moon's gravity does not "decrease". The Moon is not of uniform composition. It contains a variety of materials. The different materials have different densities. These materials are not uniformly distributed. So, the gravity field around the Moon is not uniform. If you imagine a sphere of some radius exactly centered on the Moon's center of mass, gravity will not be the same at all points on the sphere. At any given point, it is constant over time, just not necessarily the same as all other points on that sphere. So, any object orbiting the Moon is not going to travel in an ideal elliptical or circular path. It will be perturbed by the irregularities in the Moon's gravity field. These effects can add up quickly, so a Lunar orbiting satellite has to make frequent adjustments to maintain an orbit.
The farther away you get from the Moon, the less significant those irregularities become, but you don't have to go very far before another factor takes hold: the Earth.
The Earth also has an irregular distribution of mass, so Earth orbiting satellites also have to contend with similar perturbations, but to a smaller degree. The GPS satellites have to account for these perturbations. Although they don't have to make adjustments to their orbits very often, the paths of the satellites are affected enough to impact system accuracy if not factored in to the descriptions of their trajectories.