"粉饰用石膏粉" can be translated into English as "plaster powder for decorative finishing" or "gypsum powder for surface coating/decoration".
"Plaster powder" refers to the powdered form of plaster, which is commonly used for finishing and decorating surfaces.
"Gypsum powder" specifically refers to the powder made from gypsum, a mineral often used in construction and decoration.
"Decorative finishing," "surface coating," or "decoration" convey the purpose of using the powder for aesthetic enhancement of surfaces.
Both translations are acceptable, with the choice depending on the specific context and emphasis.