When I tried that on the Hollow, he a)marked experience from it and then b)copied my moves throughout the game so that he got the benefits of the blamed condition that I had put onto other people.
It sounds like the Hollow had
Better Than Nothing and
Mimicry.
Better Than Nothing does allow the Hollow player to simply mark XP whenever they gain a Condition. But having a Condition is generally a pretty negative thing, both mechanically and fictionally. Mechanically it means that other PCs should be taking advantage of that Condition to get +1's on their rolls against the Hollow and MC characters should be using that Condition to act at an Advantage. It also means that the Hollow will generally have to place themselves into dangerous or painful situations to get a Condition.
Looking at the Basic Moves, you can gain a Condition from the 10 up and 7-9 results of being
Shut Down, from the 7-9 result of
Holding Steady, or the 7-9 results of
Gazing Into the Abyss. Being
Shut Down means the Hollow would have to irritate another character or poke their nose into someone else's business to the point they piss that person off bad enough to cause that reaction. Having to
Hold Steady means the Hollow would have to put themselves into a situation that would call for a such a roll. Those are almost universally going to be tense and/or dangerous situations, which the MC should capitalize on. So yes, the Hollow can just mark XP when they get a Condition, but earning that Condition is not really meant to be fun, easy, or safe. You also can't gain the same Condition more than once. So if you already have the Terrified Condition from
Holding Steady you can't gain it again until you've cleared the first one off.
Mimicry only allows the Hollow to keep the "copied" Skin Move if they roll a 10 up with dark, and they also have to lose a move they already have on top of that. So if they were a "new" Hollow with only two Skin moves, they'd either have to lose
Better Than Nothing or
Mimicry to keep the Ghost's
Unresolved Trauma move. If they lose
Better Than Nothing they won't be able to mark XP when they gain a Condition, and if they lose
Mimicry then they're stuck with that Ghost move for a long, long time.*
In this particular case though, I don't think that the Ghost's
Unresolved Trauma move works unless the character with it has actually died. The move reads "When you project the blame and trauma of
your death..." So if you haven't
died, then I don't see how you can project the blame and trauma of that non-existent death onto someone to satisfy the requirement for that move. None of the Origin options for the Hollow really imply that they died; it's arguable if the Hollow is even alive really, and if you're not alive how can you die?
I guess I'm just echoing
Alfred and
mcdaldno here; the "brakes" for the Hollow are fictional rather than mechanical. Whatever Condition the Hollow accepts, the MC needs to make sure that all their characters treat them accordingly.
*By the way, I'm curious about something. When the Hollow replaces a move they have with the 10 up result of
Mimicry, what happens to the replaced move? Does it get crossed off the Hollow's skin book (assuming it was a Hollow skin move to begin with), or does it get unmarked and left in the book? For example, what happens if I replace
Mimicry with another move? Do I lose
Mimicry forever, or can I chose to take it again with one of the "Take another Hollow Move" Advancements?