Its likely mold from being in the shade and being dirty, bleach kills
mold you could use a garden sprayer but rinse well as bleach is hard
on fabric. The mold is likely growing on the what has fallen on the
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.