It is called wet storage stain because it occurs when a fresh zinc surface is stored in a wet environment with limited oxygen and carbon dioxide sources; the restriction in air is usually due to the items being stacked on one another or otherwise stored in close quarters.
[1] Wet storage stain is a white, crumbly, and porous substance that is a mixture of three chemical compounds: 2ZnCO3·3Zn(OH)2, ZnO, and β-Zn(OH)2.
The corrosion product is very voluminous; it is approximately 100 times greater in volume than the zinc consumed.
[1][3] To stop the corrosion from continuing the object just needs to be aired out to remove any moisture and allow the normal layer of protection to form.
Both require a thorough water rinsing afterward and do not restore lustrous surface finish if one was previously present.