Always store opened containers of maple syrup in a refrigerator or freezer. Opened containers of maple syrup will get moldy if not stored properly. Also, the natural sugars in the maple syrup will begin to form crystals very rapidly on the inside surface of the container if it is not stored in a refrigerator. After the container has been opened, and the seal has been broken, place the container of maple syrup into a refrigerator or freezer.
Non-opened containers of maple syrup have a shelf -life of many months, especially if stored in a cool room where the temperature remains fairly constant.
If mold should form on the top of the syrup, in can be restored to nearly its normal flavor by reheating to the boiling point and skimming. Add a small amount of water to replace water lost by evaporation.