Urates. Good thing she is peeing it out. Normal to do so, and normal if your tortoise is eating foods high in oxalates, oxalates will bind calcium, hence the white. Foods high in oxalates can be fed, but not every day, all day, for weeks on end. They should be fed as treats, as part of a big variety in the diet They have nutrients that are good but they also have the oxalates which can create stones later. Again, variety-variety-variety of diet is the key. Keep your tortoise well hydrated, soak and drink, so they expel anything that they should not have in their system.
And then make sure your tortoise is well hydrated. What are you using for a water source and do you see your tortoise drinking regularly? Do you ever soak your tortoise?