He can probably get through the ice with no problem, but if the ice is solid all the way through, he won't have the space to turn solid himself to grab it.

[ She has him try anyways, though. Mr. Ghostly Longcat slips under the ice like it's air, but there's really nothing he can do after he reaches the key, because he'd have to break several tons of ice in order to make room for himself. Even when he blast the area around him with Bufulas, the ice barely breaks apart before melding in with the other ice, and the key... sinks a little deeper when the ground is disturbed like that, whoops. ]