I don't know how it got there. But because of the dirt, I'm not able to use my front camera at all. The worst part is that I cannot even use iris scanner to unlock my phone. Kindly let me know if there is an easy way to get rid of that dirt.
I solved it myself using painting or cleaning brush. Just insert it inside the ear piece slot and slide it towards the front camera. Doing it twice or thrice can remove the dirt.