It is off Thien Cam Town and Cam Nhuong Commune, with locals saying the color changed deep red three days ago.
"I have not been out into the sea for the past couple days, and some tourists also said they do not dare go into the water out of concern over water quality," one person said.
A Cam Nhuong Commune official said the sea has been rough in recent days, and the waves have pushed the algae closer to the shore.
The color change is not due to pollution, they said.
In October last year seawater beneath the Cua Nhuong Bridge near Cam Nhuong Commune was in two distinct colors: yellow and grey.
Samples were tested, and the color change was found to have been caused by algae.