Why does dog poo often turn white?

First, fresh dog feces should be firm and brown, though certain foods can change its color, say to red. That said, the white feces you’re referring to is probably old feces that has decomposed, which may be the term you’re looking for, or perhaps, better yet, you can say it has “calcified.” Apparently, the reason dog poop turns white is because, after the water and any organic matter in the stool evaporates or decomposes (breaks down owing to various agents, like sunlight), it leaves behind white, inorganic calcium that may come from chewed bones or bone meal.