Probably best option is to sand it, prime it, and repaint it. Use a UV resistant paint made for plastic or UV resistant top coat, and should be good for several more years.

The green black gunk is probably algae, maybe with mildew (guessing you live somewhere that gets humid, or at least takes a while to dry from dew) so feel free to wipe it down with bleach or a fungicide after sanding but before you paint it.