iPhone shows the camera on (green signal) even the camera is not used.

A green indicator means either the camera or the camera and the microphone are being used by an app on your iPhone. (extracted from this article:--> About the orange and green indicators in your iPhone status bar).

Instead of restarting your iPhone, try to force close all the apps you opened (which you gave permission to access your mic and/or your camera), as per these instructions:--> Close an app on your iPhone or iPod touch

If the issue persists, since your iPhone is still covered by warranty, I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.