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.
Try not to fall into the common trap of wanting to replace everything on your 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 it for at least 500 rounds, then decide what you don't like and want improved. Vintage 1911's should NEVER be refinished or modified because it ruins any value they had as a collectible firearm.