Blushing happens when the release of carbon dioxide responds to moisture in the air while curing. It shows itself as a greasy film and can be removed by washing the part with warm water and a clean rag (or paper towel).
Keep in mind that this residue must be removed before top coating if fully cured, before re-coating with additional epoxy. Blushing is commonly seen in stiffer, faster-curing epoxy hardeners and is more pronounced in cool, high-humidity conditions.