Moderators are invisible for a few reasons. The fact that they can not be seen actually works out pretty well. Not knowing they are near and keeping users guessing might be enough to keep some in line. Having them show up all the time and walking around would be a distraction to their work and that is not to hard to figure out why.
There are other reason they don't let themselves go public on a regular basis but it's not important. What is important is they are out there going from space to space and others elsewere are taking reports, all doing their job and paying bill and living their life. it's a job.
As mentioned before.. Report when needed. Please don't multi-report. Don't group report. Ignore others. walk away. don't be drawn in to an argument. Report only if absolutely needed.