Use the code below to change the invalid field error message for a single form. Replace 123 with the ID of your form. Replace 'My custom error message' with your custom error message.

add_filter('frm_invalid_error_message', 'change_frm_form_error_message', 10, 2);
function change_frm_form_error_message( $invalid_msg, $args ) {
    if ( $args['form']->id == 123 ) {
        $invalid_msg = 'My custom error message';
    }
    return $invalid_msg;
}