Save Rei while it's foggy with 〜うちに
Grammar Explanation: During with 〜うちに

Jadeite captures the priestess Rei. Ami uses an ability to fill the area with fog, giving Usagi a chance to act.
- 亜美:
- 「うさぎちゃん!霧
( が出( てるうちにレイちゃんを助( けて!」- “Usagi! (Take the opportunity to) rescue Rei while it's foggy!”
Key Points
霧が出てるうちに marks a limited chance
〜うちに means doing something while a condition still holds.
出てる is casual 出ている
霧
( が出( ている means fog is out or present.The shortened 出てる is used in casual conversation.
助けて is the action to take during that window
The うちに phrase sets the timing for レイちゃんを助けて.