Moriarty's password is a paper note in Fallout 3.
Location
It can be found on Colin Moriarty's corpse, in his filing cabinet in the back of Moriarty's Saloon, or it can be obtained from Nova through dialogue with the Lady Killer perk. The password grants access to Moriarty's terminal which explains where James went after visiting Megaton.
Transcript
The password to my terminal is "lotsacaps".