If it does not specify the type of damage I would assume it to be just physical damage, but if there is a successful attack you have them make the DC 16 save to see if they are bestowed two negative levels.
An incorporeal creature’s attacks pass through (ignore) natural armor, armor, and shields, although deflection bonuses and force effects (such as mage armor) work normally against it. Incorporeal creatures cannot make trip or grapple attacks, nor can they be tripped or grappled. In fact, they cannot take any physical action that would move or manipulate an opponent or its equipment, nor are they subject to such actions.
The only difference is that it is essentially a touch attack against the target. It does not go through any form of DR though.
Just for fun I did a search on Archives of Nethys for creatures with an incorporeal touch attack. Out of the first 25 on the list they did;
Stat Damage: 7
Debuff: 2
Negative Energy: 3
Cold: 2
Fire: 2
Untyped Damage: 8
Mixed Untyped/Energy: 1
So despite my expectations, untyped physical damage is looking like the most common form of damage done by incorporeal touches.