He went Mist diving, as he put it. He was apparently in there long enough that the hallucinations still haven't subsided. He is apparently suffering other complications as well, but had enough sense to make his way to one the hospitals. He will be fine.

[But she didn't call Katara over just to talk of Mantis' foolishness, so she'll wave her after her as she turns to head further into the ship.]

But I did not call you here just to discuss that. Come along- I need input only you are in any position to provide. If I asked your brother, it would spoil the surprise.