I live north of the equator in a humid climate. The 2 north-facing windows pictured below only get direct light (lots of it) in the evening. We get a lot of rain. There is green mold growing from the windows for some reason.
I want to
The white on the stucco between the lip of the window and the stucco is unpainted white caulk that has been there for 2 years, and that I should have painted, and will once I clean off the mold.
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Mold and/or mildew (yours looks like mildew, but no big difference in regards to abatement) are both fungi and will grow where dark and damp conditions exist.
1) The cause is moisture.
2) You can clean if off easily with a scrub brush and soap, but to help prevent regrowth you should chemically kill it. A 1/4 cup bleach/1 gal. water solution will work or there are numerous consumer cleaners with bleach that will do it.
3) Take steps to keep the area dry, like rain gutters, an awning over the window, etc. Thoroughly clean, caulk and paint the stucco with a good quality exterior paint (most have mildew growth inhibitors). Chemically killing it will help prevent regrowth but be prepared to repeat if moisture cannot be controlled.
I suspect that your problem is winter.
In winter the aluminum conducts heat away from the house. Moisture condenses on the metal. (We can get 1/8" of frost on our aluminum window frames on a cold night (-40))
The air in your house is also dusty, with a substantial amount of the dust being human skin and pet dander. The combination provides enough food for molds and mildew to grow.
If this is the cause, then it will be at it's worst in late winter, and doesn't increase much during the summer.
Wash the window frames periodically with a disinfectant. Ordinary bleach works, but can be tough on clothing and nearby fabrics. Check the cleanup aisle. Often this sort of cleanup is easier in two passes. First pass just get the mold wet. Do the whole house. Next day, scrub. Giving it a day kills it and makes it easier to remove. You may ahve to try a couple of different disinfectants to find one that is really effective.
Dusting the windows edges can remove dust before it gets wet. Do this on a calm dry day in fall.
Add dust filtration to your house. If you have forced air heating, it's easiest to do this at the furnace. In a nut shell, you are just putting in a better filter system. This does the most good if the circulation fan runs all the time, cleaning the air. But try both ways.
An easy dust test is to look at a beam of sunlight slanting through the window against a dark background.
There are paints that have a fungicide incorporated. Often used in repainting bathrooms and laundries. If you wanted to do this, you would need to clean the aluminum frames with TSP, then prime, then paint. This is time consuming and finicky.
Upgrade your windows. Present prices of natural gas are low enough that this isn't a good investment in terms of changing your heating bill, but if you are in a cold climate it will make your house more pleasant.