Unless you live next door to a smelter, steel mill, or active volcano, probably mold/mildew. You could try a commercial bathroom cleaner designed to clean up those two items on a small area first. Or a mild bleach/water solution. Might have to scrub it a bit. Then rinse with soapy and/or clear water.
It's mold/mildew....not uncommon. You can wash your roof. Be careful what you use or you can severely damage it.
Most manufacturers recommend something like Spic n span, Murphy's Oil Soap, or even Dawn dish detergent and a SOFT brush. There are also commercial products out there for RV roof washing.
Make sure your roof is rated as a "walk on" roof.
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