Goliath of Greenery

You don't lose unspent green mana as steps
end. (You still lose mana as phases end.)
As long as you have unspent green mana in
your mana pool, Goliath of Greenery's
power and toughness are each equal to the
unspent green mana in your mana pool.
Otherwise, Goliath of Greenery's power and
toughness are each equal to your devotion
to green.