The most common cause of mold in a toilet bowl is stagnant water. Stagnant water occurs when the toilet is not flushed on a regular basis. Additional causes include mold spore growth in the toilet tank as well as the transfer of mold spores into the toilet bowl via toilet brushes and tools.
Mold streaks are often confused with mildew, a thin fungus that tends to develop in shower stalls and bathtubs. Mildew is easy to remove in most cases, while mold tends to adhere to surfaces and is more difficult to remove. In addition, a particular strain of mold that is referred to as "slime mold" is extremely difficult to remove, even with the application of chlorine bleach. Slime mold is a specific classification of mold in which a slimy coating develops over the mold to protect it in certain environments.
In order to safely remove mold from a toilet bowl, the entire colony must be eradicated. Mold colonies can be removed with a strong scouring pad followed by a double application of chlorine bleach. If this process is ineffective, it is always a good idea to contact a professional mold specialist to ensure a safe assessment of the issue and efficient removal of the mold.