"Please pray for my mama <3," the 16-year-old actress tweeted Tuesday.
Although she did not explain why her mother needs support, Moretz once told InStyle U.K. she was 10 when her mother battled cancer. It isn't clear whether the disease has recurred, People.com said Thursday.
Among those who responded to Moretz's call for well wishes was actress Abigail Breslin.
"Guys, please send prayers and love for my girl @ChloeGMoretz mom. Love you, Chlo. Xx."
Moretz's film credits include "Kick-Ass" and "Kick-Ass 2," "Dark Shadows," "Hugo" and the upcoming remake of "Carrie."