Welles is commissioning a movie about Kane which is a thinly veiled reference to William Randolph Hearst who, through his ownership of a media empire was incredibly powerful in Hollywood and California (if not the United States).
He was considered untouchable due to his ability to cause damage to his detractors careers and prospects,
The movie was not going to be complimentary to Hearst and so attacking him was similar to hunting the Great White Whale, Moby Dick.
Mankiewitz was writing the story/screenplay and so could be considered the captain of the ship hunting the whale, hence, Ahab.
The film Citizen Kane (released on May 1, 1941) is loosely based on Hearst's life. Welles and his collaborator, screenwriter Herman J. Mankiewicz, created Kane as a composite character, among them Harold Fowler McCormick, Samuel Insull and Howard Hughes. Hearst, enraged at the idea of Citizen Kane being a thinly disguised and very unflattering portrait of him, used his massive influence and resources to prevent the film from being released—all without even having seen it. Welles and the studio RKO Pictures resisted the pressure but Hearst and his Hollywood friends ultimately succeeded in pressuring theater chains to limit showings of Citizen Kane, resulting in only moderate box-office numbers and seriously impairing Welles's career prospects.
It should perhaps be noted that this is a meta reference to the first line of Moby Dick.
"Call me Ishmael".