In that case, I’d recommend the sanding, then a wipe down with bleach for the algae, then primer impregnated with a fungicide like Zinsser mold blocking primer (just an epa-approved example, but decent place to start). It’ll form a barrier between the raw degraded plastic and the paint (so anything existing won’t continue to grow), as well as helping prevent molds from growing in the new paint layers.

Just wait for a really dry day to do it (much as you can, you know, or if you have a garage you can run a dehumidifier in that may work too), and let the paint dry for a long time between coats to prevent any moisture trapping.