Overview. According to the American Cancer Society, chlorella (sun chlorella, green algae) is a single-celled freshwater alga that has an abundance of chlorophyll. Chlorophyll gives leafy vegetables their green color.Chlorella is commonly used in Japan to treat a number of medical conditions.verview. According to the American Cancer Society, chlorella (sun chlorella, green algae) is a single-celled freshwater alga that has an abundance of chlorophyll. Chlorophyll gives leafy vegetables their green color.