OOC:
Sorry Moss, I acknowledge the high stealth roll but it's not possible to hide unless you can turn invisible in this situation. I can't read the Mystic pdf from work, so I'm not sure on the definition of the ability. Technically the terrain around the base of the stairs is completely cleared of debris; it's like trying to sneak passed a guard in the middle of a long, well-lit hallway with no cover. No matter how well you roll, it cannot succeed unless you are turning invisible.
Even if you aren't turning invisible, I feel like this is a buzz-kill on a 30 stealth roll that is entirely for RP purposes, so I'll let it fall upon each player to decided if their character sees you or not. You don't have to declare this OOC or anything, just build it into the way you react to Moss. No mechanical benefit can stem from failing to see Moss (ie - surprise round with assassinate or something) but it's fine to include for RP purposes. Personally, I think it is a fun way to tip off people that Moss is different somehow, or to further his mysterious vibe.