The undying creature does not have a +1/+1 counter on it.
Steps
Activate Ashnod's Altar by sacrificing the undying creature, adding ({C} magic symbol) ({C} magic symbol) .
When the creature dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Activate Hexavus' second ability by paying ({1} magic symbol) and removing the +1/+1 counter from the undying creature, putting a +1/+1 counter on Hexavus.