Are you referring to the area of the tire where I've marked it? If so that's totally normal. What you're seeing here is the very edge of the tires' "contact patch". This is the area of the tire which gets "scrubbed" when cornering at speed.
2016 Ecoboost Mustang
2009 C6ZO6 - Katech Stage 2 #115
2005 Mustang V6 (Sold)
2003 Terminator (Sold)
2002 Mustang GT (Sold)