Mold growing on inside of new windows
As homes become more airtight ( new windows=less drafts and no air exchanger ) there is more of a chance for humidity to hang around and not evaporate. If you allow the moisture to just sit there the chances of mold growing is increased.  Wipe the moisture away on your windows on a regular basis. Perhaps spray them with a bleach solution as well. 

Consider adding de humidifier or some ventilation.