Sligs aro so funny!
Think about a Slig that scream "help" on your screen...ehehehehe
Ok I'm stupid...
And...about the question "What's under a Slig's mask"...in my version of Abe Exoddus (maybe the italian version could have some differences), looks like the Sligs that are sleeping in their "holes" have no mask!
I've looked careful and when they awake and try to reach a pair of pants, they have no mask!
They looks "normally" with little red eyes and green skin!