The answer to 'why is my mulch overgrown' is that the mulch has likely been left unattended for too long, allowing weeds and other vegetation to take root and grow within it. Mulch that is not regularly maintained can become a breeding ground for unwanted plants, detracting from the aesthetic of your landscaping.
To address this issue, you'll need to remove the overgrown mulch and replenish it with a fresh layer. Be sure to pull out any weeds or unwanted plants by the roots before applying new mulch. This will help prevent the problem from recurring. If the overgrowth is particularly extensive or you're unsure of the best approach, it's recommended to consult with a professional landscaper who can assess the situation and provide guidance.