Is the fluffy green stuff FA (filamentous algae) ?

Phytoplankton is the base of the food chain as Mike noted. Limiting it too much can cause a greatly reduced food chain including top line species like fish. With feeding you can short circuit the food chain (low plankton but artificially feed the fish) and still have healthy fish.

Plants (plankton and others) require 3 basic things - nutrients(fertilizer), light and temp. Remove any one and the plants won't grow. Be careful on what you do as once the limiting factor is removed growth can go crazy. For example if you fertilize but reduce the light with dye growth will slow. Once the dye wears off (the limiting factor - light) with the water warm and fertile plant growth will explode. Same with low fertility - plenty of light and temps with no nutrients and you get low growth. Add fertilizer (limiting factor nutrients) and growth can increase rapidly. Learning to manage the process is the key to good results.