dont spray the plants with anything! thats the problem they have humiditty, cut away any moulding buds before it spreds to the rest of the plant, i was gonna say fit a dehumidifier, but with the aircon it should be dehumidifying anyway, i have to put humidifiers in my grow rooms to get the humidity back up above the 60% the aircon leaves it, there is no water sitting in the aircon! i asume it is a split unit!
what is your humidity levels in the grow chambers, room?
and i stick to extraction and airflow! even if you have aircon it can have dead areas of air that need moving plenty, some species of sativas and sativa heavy hybrids do suffer with mould, normally the more common mildew, as you say this is white i feel it is more than likely to be a fungas rather than mould! the remedy is a very dry room and plenty of air circulation.

good luck

welcome to the world of endless possibilities,,,,,hydroponics