After about a month I finally got it to the range today and broke it down to clean tonight.  I have NEVER seen anything like this on a muzzle before.  Looks like cement.  I hit it with a wire brush and a couple different solvent/cleaners pretty hard wi ZERO effect.  I don't want to pick at it wi metal tools for fear of damaging the crown.

Any ideas?  Solutions?  Prob gonna contact PSA and see what they say.

Definitely not carbon. At least I've never seen white/grey carbon before. Anyway, I used the mouth of a spent brass casing and it knocked right off. I was getting a little nervous there. Will observe after next range trip if that re appears. I don't recall seeing that when I received the gun but I may not have been paying attention.

Color is unusual, betting it came from the ammo you used.  Have removed the flash hider off a mini14 and had all kind of junk under it but it was blackish like what you would expect.  Now you know it is clean, shoot a bunch and see what it looks like afterwards.