Is someone remotely accessing my iPhone?
The number by FaceTime simply means you missed a Facetime call. Just like missing a 'regular' phone call, someone tried to call you using Facetime, and you didn't answer.

There is no security risk here, no one is watching your phone or snooping on you. Someone just tried to call you. 

It is likely someone did this by mistake. On iPhone, you can easily Facetime from the Contacts app, and the Messages app, without meaning to. It is convenient, if you intend to Facetime with someone. Otherwise, it is simply and icon you never touch again!