I am not pregnate i have the mirena (levonorgestrel) iud and i just lost like a mucus plug looking thing?

Decidual Cast: You probably passed a decidual cast. Basically its the entire lining of the uterus that comes out looking white with mucus and blood. Progesterone contraception like Mirena (levonorgestrel) can increase the chance of passing a decidual cast. It doesn't mean anything bad, its more of a curiosity. If you do a search on the internet you can find lots of pictures, etc. Best wishes!