What looks like a shrimp but smaller and eats phytoplankton?

There are a few possibilities for what you're thinking of, depending on its exact size and habitat. Here are a few possibilities:

* Copepods: These tiny crustaceans are very common in freshwater and saltwater environments. They are a crucial food source for many larger animals. Some copepods do resemble small shrimp.

* Cladocerans: Also known as "water fleas," these microscopic crustaceans are similar in shape to copepods but have a distinctive "hump" on their backs. They are important grazers of phytoplankton.

* Ostracods: These small crustaceans have a distinctive bivalve shell, which is often transparent. They are also planktonic and feed on phytoplankton.

* Larvae of larger crustaceans: Many larger crustaceans, such as shrimp and crabs, have larval stages that are much smaller and resemble miniature shrimp. These larvae also feed on phytoplankton.

To give you a more precise answer, I would need more information about:

* Size: How small is it compared to a shrimp?

* Habitat: Where did you see it (freshwater, saltwater, etc.)?

* Appearance: Does it have any other distinguishing features?

With more details, I could narrow down the possibilities and give you a better answer.