Phytoplankton

In the freshwater plankton, bacteria and phytoplankton form the base of the food web. Phytoplankton— the phyto signifies plant— comprise all the free-floating "plant"-like photosynthesizers that we lump together, throwing in for good measure the unrelated bacterial photosynthesizers, the cyanobacteria, as "algae". Other kinds of protozoans that are attached semi-permanently to surfaces instead, are considered part of the biofilm.
 
You'll find the nutritious green euglenoids that form a first food for the smallest of fry on the green water page. Or you may have a purely practical interest in eliminating green water that's causing aquaristic problems.