How can this be fixed? It was like this when I picked up the 2015.
Take it back to the dealer!
It will need to be painted. It looks like it has already been lightly sanded. Did you do that?
Take it to a very good body shop or a detailer. It can probably be fixed and if it is down to the bumper a good painter or detailer can brush touch it. If you respray it it most likely will not match the quarter panels as bumpers rarely match anyways with the differing materials, but now you are adding in different paint not used on the rest of the vehicle. You would have to have it blended into the quarters which will be expensive and if it's covered under warranty they probably won't agree to the blending part.
Our painter would just cut and buff that first and brush touch if there was any flaw left. He's done far worse flaws than that and you can't even see it when he's done.
My buddy just looked at your picture and said that's polypropylene or the actual bumper material itself. It looks like bubbling to me but he's saying they look like holes since it's been sanded on, and if they are holes then there is a two part mixture that can fix and fill that. So take it to a really good body shop before you try to have it repainted. Hope that helps.
Thank you for your help. I appreciate it. I factory ordered this Mustang so I would have a Mustang free from paint defects. I looked over the Mustang before I took delivery and I still missed this. I will be taking it to a few body shops on Monday and see what they can do. Hopefully it will not need a repaint.