All datasets have a mountpoint property. You can set it to whatever directory you like. /hdd is just the default if you don't set it. e.g.:

zfs set mountpoint=/foo hdd/foo 

Answering to the question "How do I fix this?":

All datasets have a mountpoint property. You can set it to whatever directory you like. In your case, /hdd is just the default if you don't set it. e.g.:

zfs set mountpoint=/foo hdd/foo 

Some other replies so far make good points but answer to the unasked question "What should I have done to avoid this in the first place?"