It's kind of hard to tell from the picture (although I appreciate that you posted one - it's DEFINITELY better than guessing from a written description!) From your picture, I think it COULD be yeast, traub, volatiles that bubbled out of the wort, residue from a cleaning agent. . .
Bottom line is, I've seen spots like that on fermenters both home-brew and professional, and they turn out great in spite of them. (Because of them? Without being able to identify them there's no way to tell.) So let it finish out, package it, drink it, and let us know!