Py Coal Processing adds the storage tank building tailing ponds. If this storage tank reachs max capacity it can spill half it's content to the surrounding area and creating polluted ground tiles during this process. When Picker tweaks is enabled, the area where these tiles are, is colored with a green square on the map view. This green square is on the top most layer and over-colors everything.
See following screenshot as example: https://snipboard.io/GQxMOe.jpg
Can provide a save if needed.
Maybe related to this: https://mods.factorio.com/mod/PickerVehicles/discussion/64bce661ca9e9d7b6dad3462
