He's “His Eminence, Metropolitan Jonah, former Archbishop of Washington and Metropolitan of All-America and Canada”.
There is no such thing as a bishop emeritus.
In a letter, he'd be addressed "Your Eminence," not "Metropolitan Emeritus." Not even "Your former Beatitude."
In their most recent negotiations with Metropolitan Jonah, the OCA's Holy Synod agreed to change the matter of how he is to be addressed, to "His Beatitude."
The common protocol to address a retired Orthodox hierarch, is to preface his former see as "Formerly of "X", in Greek , "Proin Pittsburgh," as is the style for the retired metropolitan of the metropolis in which I reside; "His Eminence Metropolitan Maximos, Formerly of Pittsburgh." Notice, he is not a "former metropolitan," he's "Formerly of Pittsburgh."
A letter to him would be addressed, "The Most Reverend Metropolitan Maximos, Formerly of Pittsburgh." The salutation is "Your Eminence:"
I have never seen an Orthodox hierarch given an "Emeritus" status, but likewise with Roman Catholics, they were dealing with a somewhat unique situation with Pope Benedict's retirement from the Papal Office.
It is a practice within the Ecumenical Patriarchate, in some instances, to elect a retired ruling bishop to an "inactive see," to an honorary position, rather than having the bishop referred to as "Formerly of..." An "inactive see" is one wherein faithful no longer reside, and has been "inactivated" by the Synod. That usually occurs in the event of a forced retirement. The Ecumenical Patriarchate did this in 1997 when the Holy and Sacred Synod elected Metropolitan Silas of New Jersey, the respected senior member and Vice Chairman of the GOAA's Holy Synod of Bishops and close confidant of Archbishop Iakovos,' Formerly of America, but who was ailing and would not retire. The Phanar elected him "Metropolitan of Saranta Ecclecies," a venerable but "inactive" diocese of the Patriarchate. Likewise, in 1948 the Holy Synod of Constantinople encouraged Ecumenical Patriarch Maximos V to retire, electing him to the honorary Senior See of Ephesos, a "vacant, but active" (not "inactivated") "Elder See," clearing the way for the election of Patriarch Athenagoras.