I have had good results using Dettol to remove greenstuff (+ acyrlic paint) from white metal. Sometimes it will come loose overnight but I have also left minis for 3 months + they don't seem to have come to any harm. It can leave a disinfectant odour in which case I have put the minis in a sealed plastic bag with a good handful of flour, which seemed to absorb the smell. I think the minis were in the bag for a week or so.