Mold and Algae .. Starndard things where watter is dripping on the roof from the condensate drain.
Recommended treatment is Mop, Bucket, Spic & Span, and a garden hose.
Mop the entire roof, working in like 3 foot sections, starting as far from the ladder as you can
Mop it clean, hose it down and move on to the next (Still dry) section.. Till you finish while standing on the ladder
OH, you will find this improves the effiency of hte Air Conditioner too.