The Plaster of Paris should be stored in a moistureproof container as it absorbs water from moisture and turns into a hard substance (Gypsum) as shown in the following chemical equation.
CaSO4.1/2H2O+1 1/2H2O → CaSO4.2H2O
CaSO4.1/2H2O- (Plaster of Paris)
CaSO4.2H2O-(Gypsum)