How can I prevent scum forming in the water when I poach eggs?
The scum is similar to that which forms on stock; it is made of proteins which have detached from the larger protein matrix in the egg, forming a secondary protein matrix on the surface of the water. I know of no way to avoid it, but there is absolutely nothing wrong with it. If your concern is cosmetic, I wouldn't worry; it largely disappears as the bubbles pop. If you really want to get rid of it, give your poached eggs a quick rinse under warm water before drying and plating.