As usual, no one in my house was responsible for this stain. My best guess is that it is dry erase marker residue, but my daughters would not confirm or deny my suspicions.
Carpet and Upholstery Cleaner
1/2 cup white vinegar
1/2 cup ammonia
1/2 cup alcohol
1/4 cup water
Mix all ingredients in a spray bottle and shake to combine. Spray onto stain and blot with a clean white cloth.
Look at my carpet now. The stain is gone.
I have found this mixture very helpful on clothing stains also.