That is the IPV6 representation of 127.0.0.1
I wrote an earlier post on this.
MySQL 5.5 started embedding this upon initial installation. It's OK to leave it if you assign 'root'@'::1' a password. If you do not want it to have a password, then you can drop that user.
@Jakub mentioned the direct answer first, so he gets a +1 from me !!!
If you want to remove it, you could run either
DROP USER 'root'@'::1';
DELETE FROM mysql.user WHERE user='root' AND host='::1';
If you are using only root@localhost and want to eliminate all other root users, do this:
DELETE FROM mysql.user WHERE user='root' AND host<>'localhost';