If a hungry ghost monk's attack puts a damaging effect on a target (such as a bleed effect) that reduces the target below 0 hp at the beginning of the target's turn, will the monk's steal ki ability activate?
Good question; let's take a look at the PRD.
If the monk scores a confirmed critical hit against a living enemy or reduces a living enemy to 0 or fewer hit points, he can steal some of that creature's ki.
The ability does not specify how he must reduce a living enemy to 0 or fewer hit points, only that he must (or confirm a critical hit). Since it doesn't restrict how he is able to do so, as would be the case if it said something along the lines of "with a melee attack" or some such, it seems that a condition that he inflicts, such as bleed, should be sufficient, as would spell damage if he multi-classed. I can see the argument being made that he is not causing the foe to drop to 0 or fewer hit points, rather the condition imposed is, but that seems needlessly pedantic.