How do I remove this mold?
The mold growth on the underside of the toilet tank is due to condensation. This occurs warm, humid air in the bathroom interacts with the cool surface of the toilet tank. The more often the toilet is flushed, the more condensation you’ll see.
This can be prevented by running the bathroom fan more often. Ideally, replace the fan with a Panasonic WhisperGreen model. If you already have a working fan, consider installing a programmable timer switch.
The mold can be cleaned by scrubbing the area with a borax/water solution. If the area is cleaned regularly, mold growth is unlikely to reoccur, even if you continue to see condensation.