Greetings, jeetufrombikrampur.
I see that your iPhone 6s has developed an issue with the display. You're seeing pixel anomalies which seems to be growing. I'm happy to help.
About LCD display pixel anomalies for Apple products released in 2010 and later -- Pixilation can occur as any device ages. This article explains pixel anomalies and possible causes for this.
If you are concerned about pixel anomalies on your display, take your Apple product to an Apple Authorized Service Provider or Apple Retail Store for closer examination. There may be a charge for the evaluation.
Take care!