In the 19th century, glassmakers called this sort of glass "opal" (pronounced "o-pal"). The term vitro-porcelain is Sowerby's sales term for glass colors that resemble fine porcelain.
In the US, glass collectors and dealers began to use the term Milk Glass to describe opaque white glass in the 1920s-30s. The phrase stuck, and a landmark book was titled Milk Glass in the 1940s. Several glass manufacturers, including Fenton, soon adopted the term Milk Glass in their sales literature in the late 1940s and 50s (earlier, we had called it Moonstone!).