I had the same problem on my right headlight. I purchased used and it was like that when I got it. Unfortunately it was raining when I got it and didn't know it. Dealer replaced but I had to buy part. It was $217 with discount according to parts dept. It took two weeks to get it so it sounds like they are scarce. Mine is a Yukon XL Denali but I don't think that makes a difference. Also the part # has been superceded at least twice.
I still have my old one which is perfect except for leaking. It would be nice to know how to get it apart and reseal it.
If you are under warranty, definitely get it replaced. I think GM should replace free because of so many problems, quality. Mine was only 1 and 1/2 years old.
Good Luck,
Tom