The answer to 'why is my dryer vent moldy' is that a moldy dryer vent is often caused by a buildup of lint and moisture. When lint and moisture accumulate in the vent, it can create an ideal environment for mold growth. This can happen if the vent is blocked, too long, or not properly vented to the outside.
Symptoms of a moldy dryer vent include a musty odor coming from the dryer, visible mold growth on the vent or in the ductwork, and clothes that don't fully dry. The solution is to clean out the vent and make sure it is properly vented to the outside. This is a task that can often be done by the homeowner, but in some cases, it may require calling in a professional HVAC or dryer vent cleaning service.