Workshop Union Culvert

With this Hidden Moonstone farm you are relying on clearing the sheer number of mannequins in the area in order to give you a good chance for a single Moonstone to drop. This can be a relatively low effort run by using the environmental hazards to help eliminate the enemies. If nothing else, you will earn enough Ergo to be able to purchase Hidden Moonstones from Polendina after expanding his shop.

Workshop Union Entrance

Begin at the Stargazer, and go through the doorway for two quick mannequin kills and their potential drops, including Hidden Moonstones. Back at the Stargazer, make your way right and around the corner to the alley with the two mannequins lying in ambush, and the larger puppet whaling on a corpse. Dispatch the mannequins first to get them out of the way, then quickly stagger and terminate the big enemy which will drop a Star Fragment almost every time. All three can potentially drop Hidden Moonstones, Ergo Fragments, Legion Magazines, and Sawtoothed Wheels, among other items. This can be completed within a minute or less. Teleport or sprint back to the Stargazer to repeat the run again and again.

Rosa Isabelle Street Culvert

From the Stargazer, exit out into the open and around the corner to the right. Head left into the small gap, then right through the gate. This area has enough enemies on the ground to grant a chance for Crescent Moonstones to drop. Head up the scaffolding and through the building to continue the run. Teleport back to the Stargazer in order to repeat. The entire run can take three to four minutes each time depending on the power of your weapons.

Those are three of the best and easiest farming locations for Hidden Moonstones and Crescent Moonstone in Lies of P.