Without being to inspect in person, I'm going to say it's urates. Normal, as long as they are a smooth paste like texture, not gritty. If they are gritty or hard, a lot more hydration is needed.
That's urates, all reptiles "poo" in two parts. The brown part is the poo and the white part is urates which is basically their pee. It's the excess minerals and such that they are getting rid of.