Having shot benchrest rifle for 20 plus years, frosting is just s sign that the antimony is precipitating before the tin/lead is solid. That is better than shiny bullets. FWIW
The frosting easily wipes off with a light finger rub, and neither the rifle nor I are concerned. As I said, if I slow down the mould rebels and spits shiny bullets. By the way, the mould is a three-cavity.