The answer to 'why is my countertop mildew' is that mildew is a type of fungus that can grow on damp or humid surfaces, including countertops. Mildew often appears as a discolored, fuzzy growth and can be a sign of excess moisture or poor ventilation in your kitchen.
Some common causes of mildew on countertops include spills that weren't properly cleaned up, leaks from plumbing or appliances, and high humidity levels in the kitchen. Mildew can be a health hazard, so it's important to address the issue promptly.