re: Mildew smell coming from floor outlet

The conduits that are used aren’t necessarily moisture proof. I bet most floor outlets likely have moisture issues but the AC usually will remove that moisture regularly. Your rug is likely preventing the AC from removing the moisture. You can try to seal it with some expandable foam to see if that would work.
Just speaking from experience on these floor outlets. It’s not uncommon for there to be moisture within the outlet during construction because it’s below grade. Once the AC is turned on that moisture dissipates.