The green stuff is not euglena. Euglena has a red to pink color due to a pigment in the cells. What you have here is a blue green algae (cyano bacteria). Without microscopic analysis I can't tell the genus but it is most likely micrcysistis or anacysistis. Both types can produce foul odors as they decay. They also cause taste and odor problems in drinking water. They are also know toxin producers. They can be treated with copper based algicides. If the water is really green be careful because killing off too much of a bloom at one time can lead to the depletion of oxygen and a subsequent fish kill. Heavy blooms can also cause wide swings in oxygen levels which will stress fish, slow growth and possibly lead to a fish kill.