This sounds like an act of #EnvironmentalActivism to me, and not algae, as some have speculated.

#Venice authorities investigate after canal turns fluorescent green

By Barbie Latza Nadeau and Sophie Tanno, CNN
Updated 3:21 PM EDT, Sun May 28, 2023

"In 1968 Argentine artist Nicolás García Uriburu dyed the waters of the canal green with a fluorescent dye called Fluorescein, during the annual Venice Biennale. The move was designed to bring attention to ecological issues and the relationship between nature and civilization.

"The curious coloring comes as the city is celebrating the #Vogalonga boat event, created to combat wave motion and to restore #Venetian traditions and help spread attention for the environment and nature as well as the architecture Biennale, which opened last weekend. "

https://www.cnn.com/2023/05/28/europe/venice-grand-canal-green-water-intl/index.html