The answer to 'countertop mildew' is that mildew is a type of fungus that can grow on damp or poorly cleaned countertops. Symptoms include discoloration, staining, and a musty odor. Common causes include spills, poor ventilation, and inadequate cleaning.
For minor mildew, you can often remove it with a diluted bleach solution or a mildew-killing cleaner. However, if the mildew has spread extensively or keeps recurring, it's best to call a professional cleaning service. They have the expertise and equipment to thoroughly disinfect and prevent future mildew growth.