Would adrenaline increase the healing done to all creatures?
My other thought is that this would be at least a 2 card CC to remove via dmg. That might make it a little too OP for what it could do. Especially if you could field several of them, even mass CC would take 3 cards (at least with rain of fire).
But I do like the idea of a passive healer creature, but balance would be a major hurdle.