When is the last time you lifted the lid on your toilet tank? Small amounts of mold on surfaces can be cleaned with dish soap and water. I prefer to spray with vinegar 1st. Do not use bleach. The chemical does not penetrate enough and it is a harsh cleaner.
Sludge and browning in the tank is most likely from iron. I have read that food grade citric acid will clean this so it's on my list to try. You can maintain the tanks by adding 1 cup of white distilled vinegar to sit over night, once a month.