Moisture Resistant Drywall

Moisture-resistant drywall

(like "green board" or "purple board") is a gypsum panel with a water-repellent coating or fiberglass mat, designed to slow moisture absorption and inhibit mold growth in humid indoor areas like bathrooms, kitchens, and basements, but it's not waterproof and shouldn't be used in tub surrounds; it's for damp conditions, not direct water exposure.