I tend to think that I would not attempt to do anything with a piano
roll affected by mold, unless it was on the outermost wrap only.
Mold or mildew on the box I would wash with a very slightly moistened
rag until it was clean, then clean it again with a bleach-moistened
cloth.  Make sure it doesn't permeate the cardboard, only to clean it.

I had mildew on some books once that were in my storage.  I cleaned
them that way and they were fine.

Eli Shahar
Sonora, Calif.