[pianotech] [Files] Has anyone ever seen anything like this????

[Berley Antoine Firmin II]

If it isn't mold or mildew, then it might be an overzealous Mr. Fix-it 
sprayed silicone spray over the action to "help" it. I noticed a similar 
deposit once after spraying the silicone on a form-mold.
Berley Firmin
LaCombe, Louisiana