Thanks for playing!
Moving the detection for whether the slash goes up or down to the bottom sounds like a good idea, I'll try to implement that and see if it fixes some of the jank I've had with that... I initially used the player center for that purpose because having the player position at their base lead to some awkwardness with detecting whether the slash should be possible at all, but there's nothing that really requires those two things to use the same position, it'd just be a little more work to implement.
The player is just supposed to be a ghost and not a mermaid yeah :| Their visuals need a lot more work in general, I had some ideas for some particle effects and additional animations that maybe would make them feel a little more ghostly.