The Sleestak are supposed to be terrified of bright light; that is why they remain in their caves during the day, yet S'Latch seems to have no trouble walking through the jungle in broad daylight with the Marshalls. The Altrusian race (Enik's people) had no problem with sunlight, and since S'Latch inherited their traits, he is unaffected by light as well.
When Marshall is pushed into the pit ("The Hole") containing the Sleestak god, you can see him stop 'falling' just below the surface of the mist (showing it''s only a couple of feet deep).