Did you ask them what they mean by the stuff?
There isn't any yucky stuff in the egg. I need a magnifying glass to tell if an egg is fertile and it isn't yucky nor is there any taste, constituent or nutritional difference.
There are things in all eggs. Two whitish twisted strands called chelazae that hold they yolk in the middle of the egg. There is a blastoderm, which is the white dot on the yolk. If fertile, it is imperceptibly more of a halo shape rather than a dot.
There is also the inner and outer membrane that surround the albumen and the air pocket is between them.
Rarely, there is a blood spot or some shed tissue from the egg gland. That has nothing to do with fertility.
If a fertile egg is kept at a warm temperature for a length of time, cell division will begin which after several days could become yucky.
Fertile eggs kept below about 72 degrees won't get yucky.