Mildew

• The most common white plant mold is mildew. Mildew damaged leaves contain white, fuzzy or powdery blotches. Cut off infected leaves and spray the plant with a general purpose fungicide.
  
  

Other Molds

• Some rarer forms of mold, such as "silver leaf" or "honey" fungi, can appear on a hibiscus. These molds are usually grayish or yellow, but they sometimes appear white. Mold is a specific type of fungus, and a general purpose fungicide will kill these molds.
  
  

Considerations

• If you notice fine silk webbing on your hibiscus plant, you likely do not have a mold problem. Spider mites are likely the true culprit. Spider mite damage looks similar to mold growth, producing whitish leaf spots. Common chemical pesticides are usually not very effective against spider mites; kill spider mites with a soap-based insect spray.