Some individuals seem to like burying themselves more than others. And I know from experience that a tiger which spends most of its time deep within the terrarium is a pain to feed. With the moss you sometimes end up having to rip up a portion of the tank in order to surface the salamander and get it to eat.
When I had tiger salamanders I kept them in a terrarium with lots of flat rocks covering the soil. The sallies liked to dig at the base of these rocks. They could still dig deep down into the dirt but in doing so they left a large hole visible on the surface. Wiggling a worm above this hole could usually get them to come out and feed.
I'm not saying you shouldn't keep moss in the tank, as it looks nice and doubles as a substrate. Although I wouldn't rely on moss as sole means for the salamander to hide. "Hollow caves" bought from a pet store or tree bark work as good hides that don't require the salamander to dig into the ground.
I've never had this happen personally, but I've heard that some tigers will brumate during the cold months by burying themselves in the tank and not surfacing for weeks/months.
