Blue-green algae are not actually algae, they are bacteria. Green algae cells have nuclei and the pigment is distinct. Green algae are the most common algae in ponds and can be multicellular. Euglenoids are green or brown and swim with their flagellum, too. They are easy to spot because of their red eye.
Euglenoids are microscopic and single celled.
Dinoflagellates have a flagella and can swim in open waters. They are microscopic and single celled. Diatoms look like two shells that fit together. They are microscopic and single celled. Algae are photosynthetic creatures.