Sbarros is just a greater cheese amount than normal on top of a thick crust that insulates it and prevents it from melting too much. Less melted cheese is always going to be white with individual cheese strands. Some Italians call that filante or, loosely translated as 'thready.' Thready cheese happens all the time on Neapolitan bakes because the quick bake time melts the cheese less.
I can't tell you what kind of cheese this place is, but I know exactly what happened that caused the effect that you're seeing.
Advertising executive/Owner at photo shoot: Hey, the pizza looks good, but the cheese isn't pulling away, could you do the pull thing?
Photographer: Uh... well, the cheese is already beginning to cool
Ad guy: How about adding more cheese?
Photographer: Hey chef, could you melt some more pizza on this? We need a pull shot.
Chef: Sure, whatever.
Ya know,now that you said that,for this photo,I can see where the extra cheese was applied on top of the cheese used during the original bake.You can see the new white cheese pooling onto the old cheese,and the color is much different.So they added cheese after a slice was cut,put the slice back,and reheated it again.
This look is very common in slice shops I would eat at in NY.People would order slices and have extra cheese put on top.When the slices came out of the oven,the new cheese was so white,while the cheese under it was more of a yellow orange tint coming through.
Aside from that,I understand what James was describing.I will tell you,the last time I ate at Sbarros,I did not notice a filante undercooked cheese on the pie.It was nicely melted and stayed white.Not stringy like the pies that are undercooked.You could be right though,but it just wasn't my experience the last time I was there.
The sbarros cheese will take on a slight yellowy color when it cools off.It will remelt easy when old slices are reheated again.
It is why I want to try Grande,for the best,the taste,and they also advertise it melts a creamy white color and doesn't burn like other cheeses do.Hope to order some soon!