Honey, I hate to tell you, but that sounds like possible fetal tissue from a 'missed abortion,' or a miscarriage that your body didn't expel. I don't know if it's too late to get tested, but your doc could tell you. They may have clean you out and redo your Mirena. But please, please, please read this, first! http://www.virginiahopkinstestkits.com/mirenareport.html
I found the link to this site on that page, but there's a lot of things that even people who do their research don't realize about the Mirena. I had mine removed two months ago (after 4 years of screwed up emotional state, weight gain (I lost 6 lbs in two months without trying after having it out), irritable bowel syndrome...etc. Definitely call your doc. Have what you pulled out tested if you saved it, but insist that you be seen! If stuff gets left in there after a miscarriage you can get really sick, hon.