Planet Waves Humidifier

I do not want to buy another because I already have two ... Is there something I could do to reduce the mold on the humidifier?

This could occur if you use tap water or if you keep it too wet.
Some people use a few drops of bleach in the water when they see the mold, and it usually disappears. You may need more bleach if there is excessive moisture.

Once cleaned, to keep mold away ...............
You can try letting it dry completely from time to time.
Some use a hair dryer peole to get there, just do not melt.
You could try to put about 2 drops of bleach in a gallon container of distilled water. This should help keep it lighter.

Good luck.