What is the name of the white coating that can appear on improperly stored chocolate?

The white coating that can appear on improperly stored chocolate is called chocolate bloom.

Bloom is a pale, dusty, or streaky layer that forms when chocolate’s ingredients migrate to the surface. Fat bloom happens when cocoa butter separates and recrystallizes, often after temperature changes. Sugar bloom occurs when moisture dissolves sugar on the surface and the sugar later recrystallizes.

Bloom can make chocolate look unattractive and give it a crumbly or rough texture, but it does not normally mean the chocolate is unsafe to eat. It is different from mold, which may have a fuzzy appearance and can indicate spoilage. Bloom also differs from the deliberate pale decoration sometimes added by confectioners.

Proper tempering helps stabilize cocoa-butter crystals and reduces fat bloom. Chocolate is best stored in a cool, dry place with steady temperature, away from humidity and strong odors. Bloomed chocolate can often still be melted and used in baking or remolded after tempering.