Efflorescence and Byne's disease
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A white crystalline crust on calcareous specimens, and it is a symptom of the cabinet rather than of the stone.
A white bloom that is not dust
A fine white powder or crust, sometimes fluffy, on calcite, aragonite, shells and other carbonate material. It wipes off and comes back.
Under magnification it is crystalline. It is growing, and it is growing out of the specimen.
What is actually happening
Acid vapor in the air reacts with calcium carbonate at the specimen surface, forming calcium acetate and calcium formate salts. Those salts crystallize as the efflorescence you can see.
Named Byne's disease in the shell-collecting literature, which is where it was first described and where the guidance is best documented. It is a chemical attack rather than a disease, and the name has stuck.
The reaction needs moisture, so high relative humidity accelerates it and controlling humidity is most of the defense.
It is destructive: material is being converted and lost, not merely coated.
Where the acid comes from
This is the part that surprises people. The acid is usually produced by the storage.
Wood. Oak is notorious. Cabinets, drawers and drawer liners emit acetic and other carboxylic acids as they age.
Adhesives, coatings and sealants used in cabinet construction.
Degrading plastics. PVC emits acidic products as it breaks down, acetic acid chief among them, along with hydrochloric and formic acid.
A decaying pyrite specimen in the same enclosed space.
So a collector who buys a handsome old wooden cabinet has bought an acid source and put the collection inside it.
Fixing it, and preventing it
Remove the specimen from the source. That is the fix; everything else is cosmetic.
Brush the efflorescence off dry, gently. Washing carbonates is a decision to take carefully.
Then address the enclosure: inert plastic rather than PVC, sealed or barrier-coated wood, good air exchange rather than a tightly closed drawer, and humidity held moderate.
And separate sulfides from carbonates, because one of them produces what attacks the other.