The 54-year-old actress decided to quit the project, a week before filming was due to start, because of creative differences, reported Variety.
Also starring "Bridesmaids" actor, Chris O'Dowd, the project is based on the 2008 memoir of the same name by journalist Lee Israel.
The film revolves around Israel's hard times that forced her to sell letters that she had forged from deceased writers and actors.
In 1993, Israel pleaded guilty to conspiracy to sell stolen property and served six months under house arrest.
Executives of the Fox Searchlight project are now searching for Moore's replacement.
"Can You Ever Forgive Me" would have been Moore's first big screen outing after "Still Alice", that earned her the Best Actress Academy Award.