What does Nerida mean?
Associated with the Greek Nereids, the sea nymphs who were daughters of the sea-god Nereus, and with Greek neros 'water'. The name itself appears to be a relatively modern coinage (a popular Australian claim of an Aboriginal 'water lily' meaning is poorly documented), so treat the sea-nymph sense as its best-supported reading.
What does Elpida mean?
The Modern Greek form of the ancient name Elpis, from Greek 'elpis' ('hope'). The Slavic Nadezhda was coined as a translation of this Greek word.