Cleaning and painting an outdoor concrete planter

Scrub the dirt off with a brush, dish soap and water. Hose clean. Let dry and then use an exterior latex epoxy or paint.

What are the planters made of?

Use apple cider vinegar to remove the mold and algae.Is there a sealer on the planter?

Bleach that has been diluted

I would scrub it well with bleach water to get all the dirt and algae off, then spritz it with vinegar to kill any spores that are present. Since concrete is porous, let it dry in the sun for a day or two before you start the painting process.

You can power wash them or you can wire brush them. I use Rustoleum paint on pretty much everything outdoor and it works well They have a clear sealer too

https://www.doityourself.com/stry/how-to-seal-a-concrete-planter

Oxiclean ?

I DO NOT KNOW THE BRAND NAME BUT IF YOU CHECK AT YOUR PAINT OUTLET THEY SHOULD BE ABLE TO ADVISE YOU.