HackerTool/ActivityMon is classified as a Hackertool.
Hacker Tools are pieces of software utilized for subverting host or network security.
The Fortinet Anti-Spyware Research Team is currently in the process of creating a detailed description for this Spyware.
For additional information about Hackertools, and how Fortinet classifies Spyware threats please see Fortinet's paper in PDF format:
Spyware Classification and Terms.