Phytoplankton and green water is basically the same thing, the latter being used to describe the green haze that many fishkeepers don't care for, and the former being a purposefully cultured food that often contains, or is used to feed, other tiny creatures (zooplankton). The term 'Phytoplankton' encompasses more than one type, from green algae to diatoms, just as zooplankton can refer to many species. 'Plankton' covers both, at least in Melodese
My friend kept a separate culture in a sunny window, started with aquarium water. She fed some of the culture to the tank periodically. Without high light levels, phytoplankton doesn't live forever in the tank. However, it can last long enough to ensure that it's eaten, as opposed to it beginning to decompose right away. The accompanying zooplankton will survive for as long as they can find food and not become food.
If you'd rather try a product, the Rainbowfish community often uses one called Phycopure to feed tiny fry. There's another one but I can't recall the name.
My understanding is that plankton is a big part of your average freshwater clam's diet, which is probably why she was so successful at keeping them. She was also one of the most diligent fishkeepers I've ever known, which gave the clams the environment they insist upon.