It's either your boot or shoe melting on the muffler or you've been attacked by a plastic bag.

 

Use cold type oven cleaner and a not too aggressive scrub pad.

 

I agree, Boot DNA

 

It's the compensator

 

Bug Jis.

 

If you taste it, be sure to let the pipes cool. Lol. Looks like boot melt. Good luck

 

Looks like something boot or otherwise melted. Use never dull or semi chrome to clean it.

 

looks to me like it might be a bad chrome job on that specific spot and the nickel is starting to come through.

Get some nevr-dull and scrub it off completely. Then come back with some type of chrome polish over the area. If it comes back, call the manufacturer or retailer and get a new set sent to you.

 

Looks like melted boot sole to me as well. A nickel is softer that your slip on and works well to help get it off.

 

Booger???