What exactly is white stuff in urine?

Probaly sediment. Urinary sediment in an alkaline urine (non-acid) is usually due to phosphatic debris which is a normal finding. If like gravel or sand can indicate possible kidney stone disease. Occasionally lining cells of the urinary tract can be sloughed. Similarly yeast or fungus can be white stuff in the urine, but, would usually be syptomatic. Don't worry white sediment is seldom a problem.
Benign. Phosphate crystals have no clinical consequences.