What is the best way to remove mold from inside a cast iron teapot?

First of all, NEVER use soap on cast iron cookware. After cooking and while the pan is still hot, run some hot water into it and sit it back on the stove for a few minutes. If anything is burnt on, turn the burner on low. After a brief period of de-glazing, drain it and sprinkle it liberally with table salt. Wash with a cloth and a little hot water. Rinse with hot water and dry thoroughly. Allow the pot to air-dry for awhile and lightly grease the inside (if needed) with your fingers or a paper towel.

Note: a metal or synthetic scouring pad may be used if needed, as long as it contains NO SOAP. I very seldom need anything more abrasive than salt.