The answer to 'why is my steam cleaner mildew' is that a build-up of mold and mildew in the steam cleaner can cause it to emit a musty odor. This is often due to leaving water in the machine after use, which can lead to the growth of mold and mildew. Additionally, using the steam cleaner on dirty, grimy surfaces can transfer those contaminants into the machine, creating a breeding ground for mold.
To prevent mold and mildew in your steam cleaner, it's important to thoroughly clean and dry the machine after each use. Drain any remaining water, wipe down the interior, and allow it to fully air dry before storing. You can also run a cleaning solution or vinegar through the machine to help kill any existing mold or mildew. If the issue persists, you may need to replace worn or damaged components in the steam cleaner.