Complex sentence: The cause of the white appearance is thickening of the keratin layer (hyperkeratosis), which appears white when wet and hides the normal pink-red color of mucosae (the result of underlying vasculature showing through the epithelium).
Split into simpler sentences: The cause of the white appearance is thickening of the keratin layer (hyperkeratosis). This hides the normal pink-red color of mucosae (the result of underlying vasculature showing through the epithelium).