Why do hydra's appear green in color?

Hydra appear green due to the presence of symbiotic green algae called zooxanthellae or other chlorophyll-containing organisms within their cells. These algae perform photosynthesis, producing food for the hydra while also imparting a green color. The relationship benefits both organisms, as the hydra gains nutrients and the algae receive protection and access to sunlight. This symbiotic relationship is a key factor in the hydra's vibrant coloration and ecological success.