Is seeing black mold on mouth guard harmful?

Clean it: Black mold can cause localized and possible systemic infection. Clean it first by placing it in a container with denture cleanser or 2-3 drops of bleach in water. Let it soak overnight. Use a regular toothbrush under running water to scrub off the mold. Repeat as necessary to remove the mold. Leave the mouth guard in a clean container ; denture cleanser when not in use. Change solution daily.

Clean Mouthguards: It is imperative that mouthguards be cleaned everyday! most certainly, black mold can be a virulent strain that you do not want in your mouth, which is the entry to your digestive system, bloodstream and lungs. Please dispose of that mouthguard immediately.

Yuck: Why would you want to put something moldy into your mouth. Brush your mouth guard with a soft toothbrush 2x/day. Put it into denture cleaner {Polident or Efferent)1x/day as per package instructions. But before you put it back into your mouth again, take it to your Dentist and ask the Dentist to professionally clean and sterilize it for you.