"Red Euglena" are the red fresh water algae blooms per the experts here.
Per wikipedia:
Some strains of Euglena sanguinea are known to produce the alkaloid toxin euglenophycin. It is the first known case of an euglenid producing toxins. Euglenophycin is a potent ichthyotoxin that is very similar in structure to solenopsin, a alkaloid that is found in fire ant venom. Euglena sanguinea that is able to form toxic blooms that cause mortality in fish farms.
