It depends. If the defect is residual plating or etching bath salts under the soldermask, due to a board washing failure, you are FUBARed, and your only option is to reject the board. If its on top of the soldermask, try a hot very mild detergent solution and a 'pink' scourer pad (the type that's safe for non-stick pans), but the board must then be dried for at least 24H before soldering. If that doesn't remove it, try IPA and even Naptha, but don't get your hopes up!