As s/one who harvests sap and evaps it, I wholeheartedly agree that moldy syrup is not something to try and fix. The cause could be a number of things, including that it wasn't stored in the fridge after being opened. But, the problem could also have been the conditions underwhich the sap was collected and stored before it was evap'd, maybe the heat dropped while evapping dropped and the sap was left unattended for too long, the sugar content was off, or that maybe there was some silver maple sap mixed in--silver maple sap's ratio is 60:1, whereas sugar maple is 40-44:1. I know it is expensive, but better safe than sorry. My father has over evap'd sap and has had crystals form in his syrup, but, no mold or scum on the top.