What happens when Lili casts cloud serpent on a cloaked Zeratul? Can enemies see the cloud serpent while Zeratul is cloaked? Does the cloud serpent still attack while Zeratul is cloaked?
Cloud Serpent does not affect Zeratul's cloak in any way
Cloaked units always look the same, no matter what skins or mounts they use. Other gameplay elements like Abathur's Symbiote or Lili's Cloud Serpent will also not affect this appearance in any way. So enemies won't see a differerence between a Zeratul with or without Cloud Serpent.
Also automatic damage effects like Cloud Serpent will only deal damage if you are not cloaked. So the Cloud Serpent won't attack while cloaked. Burning Rage for instance works the same way, it will only deal damage around you if you are uncloaked.
An Abathur can still deal damage with his Symbiote after casting it on a cloaked Zeratul though, since it is controlled by an actual player.
Test it yourself
You can test this yourself by starting a custom game with you playing Lili and adding a Zeratul to you team. Ping him so he follows you and walk with him into any lane. Cast Cloud Serpent on him and make him move close to the lane minions. You can see that there won't be any Cloud Serpent attacks until Zeratul uncloaks himself. This works the same way when being near an enemy hero.