You need to use a child-theme. There is no way to change it by editing parent theme, because it is added by WordPress
To hide it using CSS
.must-log-in {
display: none;
}
In case you want to retain it and remove permalink, add this anywhere between opening and closing tags in your child-theme’s functions.php
function graphene_custom_comment_form_args( $args ){
$args['must_log_in'] = 'You must be logged in to post a comment.';
return $args;
}
add_filter( 'graphene_comment_form_args', 'graphene_custom_comment_form_args' );