I will be treating this fall. I did not treat last fall and almost lost a colony to nosema in late February/March. It was had to treat at that point because it was still too cold; they did not want to take syrup. By the time they did take it, the colony size was 1/2. And, I had some robbing so I figured the nosema was spreading and ended up treating everyone anyway. My feeling is that the nosema issue is still out there, lurking, waiting for the great weather to pass...so, this fall I'm going to treat.
However, if I did have that situation again, I read here (sorry, I don't remember who suggested this) to put the syrup with fumagilin in a spray bottle and spray directly on the bees. You'd have to watch your outside temps, but I thought that was a great idea!
As for mold in your syrup...sounds like the mold developed pretty quick. But if it's warm enough and the syrup is getting light, I could see it happening. Also depends on what was in the container before you put the syrup in?