What was the name of the Balrog confronted by Gandalf in Moria?

The Balrog confronted by Gandalf in Moria was known as Durin’s Bane.

Durin’s Bane was a Balrog that awakened beneath Khazad-dûm after the Dwarves delved too deeply for mithril. It killed King Durin VI in the year 1980 of the Third Age, leading the Dwarves to abandon their ancient underground city. The place later became known as Moria.

In The Fellowship of the Ring, the Fellowship encounters the creature during its escape through Moria. Gandalf holds the bridge at Khazad-dûm, then falls with the Balrog into the abyss. Their battle continues until Gandalf defeats it on the peak of Zirakzigil.

Durin’s Bane is a title rather than a personal name like the Balrog Gothmog, who appears elsewhere in Tolkien’s mythology. Smaug is also sometimes confused with the creature because both are major monsters, but Smaug is a dragon, not a Balrog.