Hydra viridissima also known as the green hydra, is a cosmopolitan species of hydra

Hydra (genus)

Hydra is a genus of simple fresh-water animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by...

 found in both temperate and tropical fresh waters. The characteristic green colour comes from cells of the unicellular alga

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

 Chlorella

Chlorella

Chlorella is a genus of single-celled green algae, belonging to the phylum Chlorophyta. It is spherical in shape, about 2 to 10 μm in diameter, and is without flagella. Chlorella contains the green photosynthetic pigments chlorophyll-a and -b in its chloroplast...

within the cells of the gastrodermis.
When disturbed, the hydra retracts to a small green blob which is easily overlooked. Gently sweeping through a clean weedy pond and allowing the collected water and plant matter to stand in a jar will often reveal a hydra re-emerging after only a few minutes.