Hi Math.. Perhaps you’re calculating wrong *hohoho*
In all seriousness, fetching a users username isn’t part of ACF but you can do it like this ( for the currently logged in user)
<?php
if(user_is_logged_in()){
$currentuser = wp_get_current_user();
echo $currentuser->user_login;
}
?>
EDIT: Sorry about the bad pun, I’ve had a bit of whisky..