The white substance in your dragons toilet is urates (urine). They have one opening (vent) which is where the feces and urine come out. A healthy excrement should be brown in colour with the urate on one end, if the excrement is covered with a jelly looking layer and is a tad watery you know your dragon is well hydrated
Dragons can only digest and defacate sufficiently if theyre husbandry is spot on. Temperatures are crucial to this.