Just like posts, a comment can have an array of different statuses, so instead of naming a hook with each status, they have transition hooks, which tell you what status it had before and what’s the new status. In your case, this might do the trick:
add_action('transition_comment_status', 'my_approve_comment_callback', 10, 3);
function my_approve_comment_callback($new_status, $old_status, $comment) {
if($old_status != $new_status) {
if($new_status == 'approved') {
// Your code here
}
}
}
Let us know how it goes?