As Andrew R Stewart said it is probably the coating coming off. It certainly looks like a coated cable.

In terms of bolts they can be easily stripped using poor tools and poor hands. Always use high quality hex wrenches and as gentle of a touch as possible and if it is an aluminum bolt or cheap steel or something just be extra extra careful with it or replace it with a good stainless steel bolt or if you need to save the weight forged titanium.

Take it back see what they say but more than likely that is coating. If they didn't replace housing and bar tape that should be done but if they offered and you refused that is bad. Anytime I am replacing cables I want to replace the housing and I want to replace the bar tape with regularity. It soaks up your sweat and since it goes down and down is your bars and your bars are hidden under the tape you have zero idea what is going on. In terms of housing it collects all the grit and grime or coating from cables and get gummed up and leave you with poorer performance or premature wear on cables. You can flush it and maybe get more time with it but honestly I would just replace it. Jagwire makes some excellent kits the Elite link being my favorite and cheaper in the long term as the "beads" (the linked housing) will last forever basically and you just replace the inner liner which is cheap and the cables and it works a treat and looks nice as well.