Why not use a custom ID as someone already suggested?
You have a variable "Breadcrumb_index = 0" or whatever
Every time you create a breadcrumb you set its breadcrumb_index to this value and increase the variable by 1 which will be the index of the next breadcrumb and so on.
Then in your overlap event you add:
Monster is overlapping breadcrumb
-----> Destroy breadcrumb
Pick Breadcrumb with lowest Breadcrumb_index (Since this has to be the next in line)
------> Monster go to breadcrumb
So even if they slowly go away, assume that those made first disappear first, then logically the next breadcrumb that the monster should go to will always be the one with the lowest Breadcrumb_index?
Or wouldn't that work?