Some buttplates were made of a black plastic called tenite.Tenite is made from cellulose acetate which is a byproduct of wood. It is also subject to a white mold which feeds on the cellulose. This mold is also often seen on very old black telephones which were molded from the same material.
If that's what is going on, I'd think that a bleach or Lysol solution to kill the mold should help, followed by rinsing and drying. Rubbing alcohol might even work better and dry much faster. However, alcohol would soften or even remove the finish if it happened to be shellac. Care is needed, and it might be best to just remove the buttplates if you have the correct tools. And you'd also want to clean and carefully dry your gun storage space to kill any spores to prevent reoccurance
