|Image from the Diocese of Savannah Website|
The motto Pax Et Bonum was a greeting St. Francis of Assisi used. Other symbols of the Bishop's Franciscan roots is the Tau cross on his personal arms, which are on the right division of the shield, and the San Damiano style processional cross behind the shield. On the left division of the shield is the Diocese's arms.
The green hat with the tassels represents his rank as Bishop.
You can read more details here