My guess, and it's a wild-assed one
Under certain circumstances, tidal forces, or a rapidly-spinning moon, would tend to stretch it into a slightly flattened shape (assuming it isn't synchronized). If these forces faded, or the spin slowed, the slow return of the moon into a more spherical shape could leave such a "wrinkle" around the equator.
That being said, I have no idea what could happen to this moon to cause such an alteration either in its tidal condition or its rotation. It would have to be slowed down a LOT.