It's about picking. Here it's better to use just use the goblin, not a separate tracker sprite. The reason it moves all goblins is because when you say tracker overlap block, move goblin, the tracker does not relate to a goblin, it picks all goblins.
There is something called a container, you can put tracker and goblin in a container and when you pick the tracker then it knows the related goblin. They are created together and automatically relate to each other.
I know in your mind though when you said for each goblin create a tracker and they seem related but actually they are not, unless you use container like above, or pick in another way like the tracker and goblin have matching instance variable IDs.
I would still start with using only the goblin object in the overlap condition to keep it simple and not this tracker object.