It's true, without routine maintenance and cleaning of your dehumidifier, the machine itself can grow mold. It's very important to clean the water holding compartment with a diluted bleach solution once a month or so. This will prevent mold from growing there, and getting blown out into the air as the machine operates.
For maximum efficiency you should also clean dirty condenser coils with a vacuum using the brush attachment, and clean the mesh filter using a soap and water solution.
If after cleaning your dehumidifier you find that it is not operating properly, double-check your room temperature. Many manufacturers recommend you operate their products at temperatures higher than 65 degrees for best results.