I'm not really sure if it should be physical or special. I kind of view bows and arrows as more physical than special, but most ranged attacks in Pokemon are special based. And if its hidden ability is Sniper, for example (a thematically appropriate hidden ability), it would be entirely useless on a physical attacker.
To me, special attacks are more energy based, while physical attacks is stuff grounded more in actual attacks. But all the (currently existing) physical Ghost attacks aren't very good. Shadow Ball is such a reliable move.
Alolan Marowak is going to run into that issue too, of a lack of Physical Ghost moves, which makes me think a Shadow Ball type physical Ghost move is coming.
Decidueye's signature move is a physical ghost type move. Who knows if it'll have a good base power though.
Also A. Marowak used some ghostly bone move in its trailer. Dunno what that was, but that might be physical as well.
