We spent thousands of dollars for new windows couple years ago. Our old windows were only 15 years old and double pane and always getting wet inside in winter so we had them all replaced with triple pane. Now these ones are getting wet only around the edge of the glass but very VERY moldy. We had the people from the window Company back after it started and checked the air and it showed comfortable but they still said it's too much humidity. We have a small monitor in use all the time and it shows comfortable so what can the problem be and if it's due to humidity why didn't the old windows grow mold. They definitely didn't. Ever.
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.
The mold is starting with condensation on your windows. Condensation happens when the relative humidity of air is near 100%.
Warm holds more water. If a roomful of air could hold 1 cup of water at 50F, maybe it can hold 2 cups of water at 70F.
It goes the other way too. If air has 1 cup at 70F (50% relative humidity) then when the temperature falls to 50F, the air has the same 1 cup of water but now it's 100% and it is condensing.
And that is what is happening behind your drapes. The air there is being chilled by the window. Perhaps the old window goass and frame had a biocide coating and the new one does not.
Cold outside air is very dry, obviously... Painfully dry... And in an older house, you interchange a lot of air with outside because they're leaky and it comes in to replace furnace combustion air. So back in the day, your furnace had a humidifier or you had chapped lips.
These days, houses are so tight a furnace will draw a vacuum, so they draw combustion air from outside, and there is so little interchange that the humidity from human action (bathing, cooking, breathing) just accumulates.
So you need to change that equilibrium somehow... Might be as simple as leave the shower door shut and let the exhaust fan do its job... Cook differently... Make sure a dryer vent is tight. Or flat out get a dehumidifier.