I think it's just polish residue, when I first started playing my brothers Fender Strat I took off the little plastic plate on the back to see if anything was corroded. There was little white powder residue caked into the corners. I'm guessing that somehow some polish got into the bridge area, probably before the bridge was put on. It'll probably come out every time you change strings, at least for a while. I'm assuming this is a new guitar, I'm sure it's nothing to worry about.