Direct from NGC's website:
White “milk” spots on modern silver coins
This form of corrosion is caused by the mint’s manufacturing process and can develop anytime, from right after striking to many years later. NCS cannot remove these unpredictable spots, but rest assured that none of NCS’s techniques cause them, either.
White “milk” spots can negatively impact a coin’s grade at NGC, but NGC cannot be liable for spots that occur after encapsulation. The NGC Guarantee specifically excludes coins “exhibiting environmental deterioration subsequent to certification. These issues include but are not limited to spotting, hazing, PVC and corrosion.”