I finally took the head set apart to see what was happening. The brown appeared to be rust, which was coming from the outside of the housing of the sealed bearing. The bearing itself seemed fine: smooth, tight and sealed, no excess play, nothing loose. The outside of it, where it seats in the fork, was lightly covered with rust, to the point that I could not read, or even find, the etched labeling on it. The stuff oozing out the top of the head tube seemed to be a rust colored slurry of the assembly grease, rust, and sweat. There was a lot of scale inside the stem and spacers, presumably dried from sweat dripping on the top of the stem and running down inside. The steerer tube is indeed carbon fiber, so the only steel anywhere in the head tube was the bearing housing. Comparing the top bearing to the bottom bearing, the top bearing housing was much rustier than the bottom one, again presumably from sweat getting to it. The assembly grease on the bottom bearing was not rust colored. It was the translucent traditional "grease color". I'm thinking maybe I should do something to seal the pre-load cap so it doesn't collect sweat so much.