How do I source, remove, and prevent green mold on my aluminum windows?
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.