The animals appear to be Glaucus atlanticus, a type of sea slug that is also known as the sea swallow and the blue angel among other nicknames.
The creatures were found on Fish Hoek Beach, where they were photographed by Maria Wagener, who runs a Facebook page about the shore and its sealife.
Ms Wagener said she found around 20 on the beach, adding that she had never seen the creatures before, despite living near the beach for most of her life.
Suspecting that the sea slugs might be dangerous, she didn’t risk picking one up.
“I pick up starfish all the time and put them back into the sea but I had a feeling that these would have a sting," Ms Wagener told The Sun.
She added: “I probably would have put them back in the sea if I’d had something to lift them, but no, I didn’t touch them!”
Although they look beautiful, with finger-like cerata in different shades of blue, the creatures can deliver a powerful sting.
The animals absorb the stinging cells from their prey, which include the venomous Portuguese man o’war, and store them in the cerata. They use the tentacles as a defence mechanism against predators.
The painful sting can lead to nausea, vomiting as well as acute allergic dermatitis, which can appear as itching, a rash or skin lesions.
The sea slugs have previously been found stranded on on beaches in Australia as well as in the US, and can be found throughout the Atlantic, Pacific and Indian Oceans.
The aquatic creatures float upside down carried along the sea’s surface by the winds and ocean currents. Blue on their underside and silver on the other, the animals face upwards in order to camouflage against the sea.
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies