I don't have enough information to say for sure. It could be fiction, or it could have roots in reality that have been adapted or exaggerated for a particular purpose.
The story of Anger Sierra might focus on Sierra's battles with her own anger, how it shapes her personality and the challenges she faces because of it. It could also involve her growth and transformation in dealing with this emotion.
I don't believe it's true. Most of the time, when a story has a name like that, it's made up to capture our interest and take us on a fictional journey rather than being rooted in reality.
In Sierra Vista, there was a story about a hiker who got lost in the nearby mountains. As night fell, he started seeing glowing eyes all around him. He felt like something was stalking him but couldn't see what it was. In Sacramento, there are stories of a cursed alley. People who walked through it at night often felt an overwhelming sense of dread and some even claimed to have been physically pushed by an unseen force.
Since we only have the title 'robert california tells halloween story', it's hard to know exactly who he is. He might be a local storyteller in the context of the story, someone known for sharing spooky tales during Halloween. Or perhaps he's a character with some special connection to the events in the story, like he has a past experience related to the spooky elements in the Halloween story.