Your bees will be fine. No matter how you feed or prepare syrup, if it is left long enough, it will start to grow
I have never seen bees take syrup that had gone bad. Just because there is some mold, doesn't mean the syrup has gone bad.......yet.
Some people try boiling the water first in an effort to kill any bacteria that is in the water, but this is of little help. As soon as the bees start drinking the syrup they introduce bacteria from their mouths. Some feeder are worse than others. Boardman feeders are exposed to sunlight which makes them mold faster. Any larger quantity feeder, such as a hive top feeder, allows the syrup to age longer and bacteria to grow. Of couse any feeder that allows dead bees to end up in syrup doesn't help the situation.
Adding cider vinegar seems to help the syrup last longer, as does HBH.