I'm not one hundred percent sure what happened here. I think that's a plastic cutting board that they tried to cook the pizza on. Whatever it is, you're not supposed to expose it to four hundred degree heat. I'm just surprised nothing truly horrible happened. I guess there is the loss of the pizza. That's pretty horrible. You'd know that if you ever lost pizza, but someone must have had some slices already (somehow) because that is not a full pizza.
I wouldn't know where to start cleaning this. Hopefully it just pops off the oven, but they might have to turn the oven on again to turn the cutting board back into goo. Next step is to use a squeegee to scrape it all off. Suffice it to say, this is not a situation you want to be in.