The answer to 'why is my grout moldy' is that grout is a porous material that can easily trap moisture, making it a prime breeding ground for mold and mildew. Grout mold is a common issue, especially in bathrooms and other high-moisture areas. Factors that can contribute to grout mold include poor ventilation, excessive humidity, water leaks, and improper cleaning or sealing of the grout.
Grout mold can appear as black, green, or even pink discoloration in the grout lines. It's not only unsightly but can also be a health hazard, as mold exposure can cause respiratory issues and other problems. To address grout mold, you'll need to identify and address the underlying moisture issue, clean the affected areas, and consider re-sealing the grout to prevent future mold growth.