Why is sodium sulphate hemihydrate called as "plaster of paris"?

Do you mean semi-aqueous gypsum calcium?
Is called a semi-aqueous gypsum calcium sulfate, as the formula: CaSO4 * 1/2 H 2 O molecule of water is half (hemihydrate), contains sulfur (sulfate), and calcium (Ca).
Gypsum is a common name because it is used in construction.

Calcium sulphate hemihydrate( CaSO₄.1/2H₂O) is also called as the Plaster of Paris because of the reason that Paris, during 18th century, became the major center of this compound's production.
Plaster of Paris is a hemihydrate of Gypsum and gypsum ores were available in huge amounts around Paris.
Even now, Paris is a major site for the production of this hemihydrate of Calcium sulphate.
Hence the name Plaster of Paris.