There's a strange residue on the inside of my miacup, near the top. It's kind of yellowish, hard to see, but I can feel it. It appeared a few months ago, but I've kept using the cup -- the residue hasn't gone away, nor has it gotten worse. I always wash the cup with soap and rinse, but I don't always boil it between cycles (though I have been since the residue showed up -- I also tried boiling with baking soda and soaking in diluted vinegar). I'd try scrubbing it, but I don't have an old toothbrush or anything lying around. I haven't noticed any accompanying health problems, but is this something I should worry about?