Silvering is small air bubbles between the thermal adhesive and the printed substrate. These occur when the paper has greater roughness, insufficient pressure in the lamination nip or the laminating temperature is too low. Generally, the paper is already printed, so that cannot change, but jobs for laminating should be a smoother paper as the ultimate surface appearance is the thermal laminating film.
What to do:
Increase the pressure and bring the lamination nip temperature up until it goes away. If you are the limits of what the laminating machine can do, use a thicker laminating film because generally the thicker films have a greater thickness of thermal adhesive on them.