The adaptation is based on Hans Christian Anderson's 1837 fairy tale, rather than Disney's take.
Illustrations in the original book show the mermaid with a golden mane.
"I'll actually have blonde hair," Moretz revealed.
Screenwriter Richard Curtis, who also penned Love Actually and Notting Hill, is still working on the script.
The Universal production was originally to be directed by Sofia Coppola but she stepped away from the project because of creative differences.
A director has not yet been appointed to the project, but Moretz said it will be a fresh take on the theme.
"[It's] a modern, revisionist tale, progressive for young women in this day and age."

- nzherald.co.nz