True, both of the red and white phosphorus is a pure substance.
Both red and white phosphorus is made of phosphor element but they have a different structure which causes them to have a different color. This phenomenon called allotropes when a chemical element has two or more different form.They are not compounds as they are only made of phosphorus. A compound should be made by at least two different elements.
The atomic mass of Cs is 132.9. So the mole number of 675 g cesium is 675/132.9 = 5.08 mole. Then the answer should be 5.08 mole.
All elements are pure substances.