Druidry is a religion or philosophy that often promotes harmony and worship of nature, and respect for all beings, including the environment. It was reconstructed in the 19th century after the Druids of Celtic cultures.
Green and Sea witchcraft is secular. It can fit into many paradigms, and is focused around nature/plant craftings, and sea-based craftings.
So a Druid can be a sea or green witch, but not all such witches are druids, just as not all druids are such. In fact, few druids consider themselves witches at all.