I replaced the rear pinion seal on my 1990 4x4 V6 pickup to fix a leak caused by a blocked breather valve. It went together fine and seems to drive normally. Looking under the truck, I noticed that the rear drive pinion companion flange was covered with white crystals. If it were cold, I would swear it was frost. The areas that are covered no longer have paint on them. It looks like the part was stripped chemically. The crystals are also lightly sprinkled around the surrounding area. It almost looks like liquid was splattered which is now reacting to the metal? I replaced the gear oil in the diff when I replaced the pinion seal, but I can't think of any other liquids that might be in that area. Any ideas what it might be? Should I worry about corrosion? I've uploaded before and after pics to show what I mean. Everything white is new.

Before

After

After

 

If it were mine I'd brush it clean with hot soapy water using a healthy dose of Dawn soap. Then rinse well with water. then see if it comes back. It looks like residue from a cleaner after it dried.
Old toothbrushes work great in tight spots.
I hate bare metal. I paint or powder coat whenever I can