How should I clean these plastic items?
That looks like lichen.  On plastic, you can probably remove it with just soapy water and a bristle brush or nylon scrubbing pad (but don't use something abrasive enough to scratch up the plastic or it will provide more surface for the next round of it to adhere to).

It may help to first spray the lichen with vinegar, full strength with a little added soap as a wetting agent.  Wet the lichen well and let it dry.  Give it a few days for the lichen to die, then clean it as above.  I've also had good luck spraying it with household (sudsing) ammonia at the concentration recommended for cleaning, then giving it a few days before washing.  For really stubborn cases, do either treatment regimen and if some remains, repeat with the other (but don't mix vinegar and ammonia to try to combine both in one treatment; they will neutralize each other).

If you have a pressure washer, that should remove it with no special treatment, but start with a lower pressure (wider angle) nozzle and some distance to use the minimum pressure needed for cleaning without damaging the plastic.