How to turn off the iSight indicator when I'm using the Camera?
No, you cant.
Fortunately (or in your case unfortunately) there is no way to simply hit a switch that turns off the light. The problem is that the iSight camera's light is set to come on whenever an application uses it.
It's partly a security feature built-in for your own convenience. Think if it like the "On Air" sign in a way.
Depending on your model of mac (you didn't mention one specifically) it may be possible to physically disconnect the LED light from the controller, rendering it offline until plugged in again. However, for the majority of new Macs this may prove a challenge.
If your bezel doesn't happen to be white metal (think MacBook Air), you could go over it with a black permanent marker. Something that greatly reduces the light emitted from the iSight indicator.

At this time, there is no software to disable it. You would either:


Quit the Application using the camera.
Inhibit the physical connection to the indicator.


Hopefully that shed some light on the situation in 2018 as similar questions have attracted a lot of attention over the years. Even I have searched for a solution many years ago but have learnt since to ignore it.

Let me know what model you have and I might be able to be a little more specific in my reasoning (much has changed in the physical layout of the bezel). Let me know how you make out or if you come up with a better solution, I'd be very interested myself.