The Environment Agency has confirmed the potentially toxic substance is present in some waters in Alexandra Park, with particularly high levels in Shornden Reservoir, near Clarence Road.
This algae can produce a scum that is toxic to animals and can harm people. Hastings Borough Council is warning people and their pets to stay out of the water and have put up signs around the affected areas to warn members of the public.
A council spokesman said: “Hastings Borough Council will continue to monitor the waterways in this area. The algae can spread to other waters in the park, so please be vigilant. The algae is bright green and looks like a fine scum floating on the surface.
“Fortunately this natural phenomenon will go away in time when temperatures drop significantly but this is unlikely to happen until the winter.
“Please forward any new sightings of suspected blue green algae to the Environment Agency on their incident hotline (24hrs service) 0800 80 70 60.”