50 Phosphorescent White / Glow In Dark Opaque
BrickLink: Glow In Dark Opaque
Introduced in 1990 as the classic Castle ghost, and used until 2006, when it was replaced by 294 Phosphorescent Green.
BrickLink's name for this color is a little misleading, since it's more translucent than opaque -- the modern 329 White Glow is more opaque. Also, rubber or soft-plastic parts that are technically 294 Phosphorescent Green, although they look close to opaque.