Little Red Dot

That dot should be Orange and it may look like Red. That Orange dot informs the user (your) that an app is using your microphone. Next time you make a phone call then take a look and you will see that the Orange dot will appear. This is because you use the microphone during a phone call. Same results when you are using FaceTime or any app that uses a microphone. This is a user awareness feature to let you know that your microphone is active.

Axel F.

There is a green dot that indicates your camera is in use, and an orange dot indicating the microphone is in use.

An orange dot at the top of your screen indicates your microphone is being used.