"There was what I thought was white paint all over the buttplate. Some scraping with a toothpick and then work with a toothbrush got it off. Thought I would have to replace the buttplate. The white stuff came off much easier than even latex paint would have. I am thinking it was dried out white lithium grease. The toothpick was softer than the plastic buttplate, so it appears to be clean , no damage to the buttplate."
That white stuff is mildew - for some reason it seems to like older hard rubber buttplates, but as you discovered doesn't seem to hurt them. I clean it off with Balistoil and an old toothbrush.
