Detail of larger mosaic. I think this picture was taken when it was in very clear light, on other pictures, taken in 2019, there is artificial light. A viewer informed me this is Arethusa. From the Wikipedia: "In Greek mythology, Arethusa (/ˌærɪˈθjuːzə/; Greek: Ἀρέθουσα) was a nymph and daughter of Nereus (making her a Nereid), who fled from her home in Arcadia beneath the sea and came up as a fresh water fountain on the island of Ortygia in Syracuse, Sicily."