As Clone Shell's first ability resolves, you must exile one of the cards you look at, even if none of them is a creature card.

If you gain control of another player's Clone Shell, you can't look at the face-down exiled card. However, if Clone Shell dies, you turn that card face up and, if it's a creature card, you put it onto the battlefield under your control.

As Clone Shell's second ability resolves, if the exiled card is not a creature card, it simply remains in exile face up.