In the Greek the term is written in past tense (“reigned”). That is not to say that his reigning has come to end by the time the servants sing this song (that cannot be since he is the “Almighty”), but it is to say that the events relating to the fall of Babylon as recorded in Revelation 18:1–24 unfolded as they did because “the Lord our God the Almighty” was reigning supremely, even as the prostitute was doing her thing on earth. Here is the echo of Revelation 17:14: though the prostitute and the kings team up to make war on the Lamb, “the Lamb will conquer them, for he is Lord of lords and King of kings.”
6 Then I heard what seemed to be the voice of a great multitude, like the roar of many waters and like the sound of mighty peals of thunder, crying out, “Hallelujah! For the Lord our God the Almighty reigns.