Macbook webcam only shows black and green LED is always on
Based on the description and the comments, this sounds like a hardware failure, not malware.   The iSight camera and/or cable has most likely failed.  Replacing it should be a fairly simple task to a tech to accomplish

The first thing to do when having hardware difficulty of any type is to run Apple Hardware Test (AHT).  Simply hold down the D key while booting from a powered off state and the AC adapter connected.

Since AHT diagnosed an issue and you are under AppleCare, it's best that you take it in.