This is the Living Mana HS Druid Card from the Journey to Un’Goro expansion due for release in April.
It’s a Druid class spell and the card reads: Transform your Mana Crystals into 2/2 minions (Mana Treant). Recover the mana when they die.
You can get Living Mana by opening packs from the Journey to Un’Goro set.
*Interesting spell, could swing tempo into your favor.