I've seen the same thing in mine awhile back now that you mention it. I was shining in a flash light (one of my favorite hobbies) and saw these things. Can't remember if they were swimming or on the glass but they are REALLY thin, small and yeah 1/4 - 1/2 inch long if i remember and red.
They aren't bristleworms.. never had any problems in my tank but thought it was interesting one night... haven't seen (or looked for them) again as it was kind of out of sight out of mind.