Counter target spell. Create an X/X green Ooze creature token, where X is that spell's converted mana cost.
3/14/2017 If the target spell is an illegal target when Mystic Genesis tries to resolve, Mystic Genesis will be countered and none of its effects will happen. You won’t get an Ooze token.
3/14/2017 You may target a spell that can’t be countered. When Mystic Genesis resolves, the target spell will be unaffected, but you’ll still get an Ooze token.