I'm with Rob, that looks to be some sort of mold. The paper sleeve or inside the album cover should show some staining as well. After Hurricane Rita in 2005, I was forced to eventually discard some vinyl. They too had mold that I couldn't find a way to remove......not all mold is the same!
Not sure how often Savage Clone comes around, but you may want to send him a PM with a link to this thread. Working all those years at a record shop, I know he would have the best advice for you.
Savage Clone - Stagnation is the sign of true squaredom