I thought of 2 possibilities when reading your post... either residue/build-up from the wort/fermentation or mold (or even bacteria). You may never know without looking at it through a microscope, but since it is above the water line, even if it was mold/bacteria, your beer probably isn't infected.
I've never had that in my airlock, but when I ferment in my chest freezer, I get fermentation/wort residue build-up on the inside of the chest freezer that gets carried off with the CO2 from fermentation. Just to be safe, in case you don't do this, you should always put some sort of sanitizer solution in the airlock... or even cheap vodka. I typically put a weak iodophor solution in it, but occasionally use vodka.