Edit: Correction for warm climate.
Mold/mildew results from a combination of dirt/dust/pollen/pet dander, skin cells, bug crap combined with moisture. Inside, you can filter to reduce this. Outside, you're have to clean more often.
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 have 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. Outside, try using a shopvac with a brush tip. This material is very fine. Some is going to go right through your vacuum.
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. You may be able to get this as an additive, allowing you to use any latex paint. You may want to use a colour that matches the colour of your mold colony. This doesn't eliminate the problem but it makes it less visible.
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. In a cooling climate, better windows may save enough on air conditioning costs to pay for themselves.
If your are in a cold climate: 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.
Options. In addition to the ones above, these may help:
- Add dust filtration to your house. (Only if indoor problem) 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.
- Dust the windows to remove accumulated organic matter.