Excess Mana

Excess Mana is an uncollectible druid spell card, obtained by playing Wild Growth, Pilfered Power, Overgrowth, or Astral Communion when the player already has 10 Mana Crystals. It can also be generated by playing Wild Growth when the card's effect would cause you to have more than 10 Mana Crystals (such as by using Innervate on turn 9 with 9 full Mana Crystals to get to 10 total mana and then playing a Wild Growth for 0 mana).

Excess Mana is intended to provide a useful alternative effect for when playing a card with 10 Mana Crystals would otherwise provide no useful effect. For this reason, other cards such as Nourish or Grove Tender never produce Excess Mana, despite their ability to grant extra Mana Crystals, since they have alternative Choose One effects or additional effects that are still useful at 10 Mana Crystals. This is also true for Greedy Sprite.

For more information, see Wild Growth, Pilfered Power and Astral Communion.

Lore
has been crystallized as a consumable in Warcraft before. Most significant would be the mage's Mana Gems, used like an additional potion to restore their mana during long fights, and this card's art in fact depicts a mage's. Druids are however not known to have any such gems or abilities.

Trivia

 * The artwork for this card comes from the World of Warcraft Trading Card Game "Heroes of Azeroth" series, depicting the spell.
 * This card's art appears to depict a female troll engaged in some kind of magical work. However, the relatively scholarly setting complete with several tomes clearly does not fit with troll tradition, perhaps reflecting this troll's departure from the ways of her kind.