Since you're using built in movement, the monster is likely overshooting the cursor. Change Y("Monster") == Y("Cursor") event to:
*Y("Monster") <= Y("Cursor")+ 2(or whatever margin of error you want)*
*+Y("Monster") >= Y("Cursor") - 2*
Set Alterable value("Monster") to 0
Set Y("Monster") to Y("Cursor")
Destroy Cursor
Alternatively, do both the movement and checking one pixel at a time in a fastloop.
"Omg. Where did they get the idea to not use army guys? Are they taking drugs?" --Tim Schafer on originality in videogames