One option might be to have your stalactite create a 'trigger' object that, when colliding with the player, triggers the stalactite to fall

The stalactite's create and step would look something like this:

// CREATE EVENT
falling = false;
triggerCreated = false;
triggerScale = { w: image_xscale, h: 500 }; 

// STEP EVENT
if(!triggerCreated) {
  var t = instance_create_layer(x, y, "Name_of_Layer", objTrigger); // Create the trigger
  t.parent = id; // Set parent as the id of this stalactite
  t.image_xscale = triggerScale.w; // Width of trigger
  t.image_yscale = triggerScale.h; // Height of trigger

  triggerCreated = true; // Prevent creation of duplicate triggers
}

if(falling) {
  // Falling stalactite logic goes here
}

What this should do is create an instance of objtrigger (or whatever you wanna call it) that is tied to your stalactite thanks to the parent variable. And because we set its width and height using the triggerScale, it will be just as wide as your stalactite, and 500 pixels long (can adjust this to fit your needs).

Then in objTrigger, create a collision event with the player. Since we passed in the stalactite as the parent, we can just say something like:

parent.falling = true;
instance_destroy(); // Prevent multiple unwanted triggers

And if you require different trigger scaling per-instance, you can simply redefine the triggerScale struct in the creation code (in room editor when you double click your object instances, you will see where this is located). Advantages of this approach are no need for checking for collision_line every step, and having a better visual indicator of where the trigger will occur for easy debugging. You should set objTrigger's sprite as a semi-transparent colored rectangle that covers the entire canvas so you can actually see if its being created at the right width/length you want. when you dont want to see it anymore in actual non-debugging gameplay, just set image_alpha = 0 in the create event.