Some effects (most notably, the standard effects of Time Passes) discard Power Cards from play because they interact with a specific card that's in play or with cards in play in general. In that case, put the Power Card(s) from play into your discard pile. You can't discard a Power Card from play unless otherwise instructed to do so.
However, the Spirit still needs to pay any costs associated with using the Power, whether that's a mandatory "Cost to Use" (e.g., for Absolute Stasis) or a cost associated with a particular choice (e.g., for Hazards Spread Across the Island). You can pick a Power Card with a Cost to Use that you can't afford; in that case, you fail to use the Power and then Forget the card. Tags: Cost To Use, Resolving Powers, Ethereal Conjunction, Absolute Stasis (Fractured Days Split the Sky), 3 more...
However, the Power Card will be Forgotten before you have a chance to use any Powers in Slow Phase, so it won’t help you hit the Elemental thresholds of those Slow Powers. Tags: Elements, Ethereal Conjunction