He was an ancestor of Círdan.
Other versions of the legendarium
Elmo was developed to explain the reference in The Silmarillion that Celeborn was a "kinsman of Thingol". One of the solutions was to establish Celeborn as the son of an unimportant brother of Thingol.
It therefore remains unsure if Elmo would have a place in the Middle-earth canon, as his position in the royal Sindarin family tree disappeared. Slightly later writings however make Círdan a descendant of Elmo, making him a kinsman to Thingol (and Celeborn). Indeed Círdan had silver hair, a trait which otherwise only ran in Elwë and Olwë's family line.