What type of buttercream did you use - sometimes this is caused by salt that was not disolved thoroughly
Wow have never seen this before. Hope you find out what caused it.
This kind of thing happened to me once, when I was using buttercream. I had piped the buttercream onto the cake, and the heat of my hand around the bag had slightly melted the butter in the buttercream, and it was slightly softer than it should have been. Anyway, the next morning I had quite a few white spots. I think it's like the grease in the butter that re-sets into those white spots.
Not sure it was definitely the same thing that happened to your cake, but just thought I'd share.
This has happened to me more than once. I use crisco so I don't believe it is the butter and have used salt and then no salt and it still happened. Looked cool on a tool chest but my leaves looked like they were diseased. LOL Hope someone has an answer for us.