I let mine soak in a bleach solution for around 30 mins, that should get rid of it all, if not then let them soak a little more. After that I rinse and rinse then do a final rinse in some water conditioner then put them all back in and they look like new.
When I had plastic plants, I used to soak them in bleach too. But the really though algae it didn't shift at all and I just had to bin the plants. It also helps to scrub (with a pot cleaning brush) under really hot water.