Painted metal roofs, especially those with the industry leading PVDF finishes which are sold under the Kynar and Hylar trade names, are very resistant to streaking and staining from atmospheric fungus and subsequent biological growth. However, some climates will eventually cause mildew or other things to grow on just about any surface, even glass. Additionally, if trees overhang a metal roof, those trees can drop sap on the roof and that sap will hold dirt as well as eventual biological growth.
Always check with your roofing manufacturer before proceeding with any cleaning process but most metal roofs can be scrubbed or lightly power washed to clean them. Additionally, there are products like Wet And Forget and Roof Wash which can be sprayed on the roof that will naturally kill biological growth. Again, check with your roofing manufacturer to ensure those products are compatible with your roof.
Kynar 500® is a registered trademark belonging to Arkema Inc.
Hylar 5000® is a registered trademark belonging to Solvay