Hi there...I am not a GP, but I believe spotting is a common side effect of Mirena, which could be mixed in with your discharge. Also, it could be a period, once mirena is inserted your cycles become less frequent/can disappear or become lighter.
I would talk to your Doctor.

As a side note, I had Mirena removed in January and it really messed up my cycles and me, I also had the spotting with the mirena in and out(apparently when you ovulate sometimes you can spot), so it could be related to ovulation.

My recommendation is to get the Mirena removed, there are other forms of BC, that are less damaging than the mirena!
BEST OF LUCK!

Many of the women on this forum (including me) have had severe autoimmune and neurological reactions to the Mirena, along with massive hormonal disruption. I have had mine out for almost 4 years and am still not back to normal. Please consider having it removed just to protect your health! And feel free to spread the word. BTW, many doctors refuse to acknowledge the damage the IUD can do, although there are some who do. My Dr. tried to talk me into keeping it, but my instinct told me something was wrong. My body was not happy with it, even at the beginning when the symptoms were easy to overlook. I also had some spotting and cramping with it initially.

Good luck in your decision :)

Best wishes,
Diana