Reef Olives are a fruit, presumably similar to land olives that grow on reefs.
History[]
When Serafina and Neela met up with Ling, in Deep Blue, to go to the Olt River, they mention bringing reef olives with them.
in Rogue Wave, Becca mentions finishing her last handful of reef olives in the morning before she went to the market, meeting Astrid. At the end of the novel, the merls, minus Ava who went to the Mississippi, ate reef olives along with their dinner.