In the Nicene Creed what does "He will come again to judge the living and the dead" refer to? Since we are judged at the moment of death, I take this to mean: when Jesus returns, those that are still alive in a worldly sense will be judged on that day. The living and the dead is referring to the souls of those people. The living is one that is living in faith as a Christian, and the dead is one that is dead to sin.
In all three ecumenical creeds (Apostles’, Nicene and Athanasian) Christians confess what Scripture teaches: “…Christ Jesus, who will judge the living and the dead…” (2 Timothy 4:1).
As you noted, when people die, judgment takes place (Hebrews 9:27). When Jesus returns visibly on the Last Day, he will make public what those judgments at death were, and he will pronounce judgment on those who are alive on the earth on that day (Matthew 25:31-46).
With reference to whether people died before Jesus’ visible return on the Last Day or if they are alive on that day, Jesus will “judge the living and the dead.”