I heard your panic, but can't help. I used it only as a top layer for sheet-size carrot cakes and
then marked it off for the caterers and piped a daisy on each piece.
Five half-sheet cakes sat in Mom's refrigerator for two days. No degradation.
I check Rosie's website and one poster mentioned that it didn't crust up but she actually DIDN'T want the icing to crust, so she was happy. She did have an issue that it was too light to mask red velvet cake.
Another poster said to add additional powdered sugar to get the icing to crust.