The Terrain-Mercury is a subset of the Terrain preset. The Terrain-Mercury eliminates all the mercury-detox related frequency from the Terrain preset. The Terrain preset has the mercury-detox frequencies. The mercury frequencies don't release the mercury from your fillings, but instead, help your body detox mercury. The reason people asked for the mercury-free version of the Terrain preset was that they still had mercury fillings, and the people thought the detoxing of mercury would be very painful (it could be) and not something they want to engage in at this time.
The more mercury you leave inside your blood/tissue/organs, the harder it is to get healthy.
For more details, please check the link: