I believe those are bluing salts that leached out from under the rear sight after refinishing. I would think that Hoppes #9 should get rid of it.
Avoid the temptation to replace everything on your brand-new 1911 just to make it "better". Know what you're changing out and why. You may spend a lot of money fixing things that weren't broken to begin with. Shoot at least 500 rounds through it first, then decide what you don't like and want to improve. Regarding vintage 1911s, pre-1946 pistols are highly collectible in original, unaltered condition and should NEVER be refinished or modified as it completely ruins their monetary value.