What do I do with mildly fermented maple syrup?
Yeah, syrup can ferment + convert sugar to alcohol. It has more of a tendency to do so if the sugar content is lower -- I tend to err on the side of overconcentrating my syrup.

You can try boiling it for a while to see if the alcohol boils off + if the flavor is OK then use it... but I'd boil down a bit more first, to make sure the sugar content is back up to standards. Either boil until the boiling point is 7 degrees F higher than the boiling point of water at your altitude, or boil until the syrup "aprons" (e.g. starts to drip in a sheet rather than discrete drops; a flat edge of a metal spatula works well), with the former being more accurate if you have a good thermometer.

If the flavor remains after boiling, then try using in recipes -- perhaps in brownies/blondies or with ice cream.