I'm not entirely sure I fully understand what you did or tried to do to get rid of that new post indicator in Peripherals, but let me tell you what I think happened. The thread was moved from Peripherals to Mac OS X Applications because the subject matter fit better there; as part of the move a temporary pointer was left behind in Peripherals.
As long as the thread in question is the last one posted to in Peripherals, you see it listed at the Forums List level, and when you click on it there you access the thread in its new location in Mac OS X Applications. Because you didn't get there by clicking on the pointer in Peripherals, that pointer behaves as any other thread with a new post and maintains the red new post marker. You get rid of it the same way you do any other new marker, by clicking on its link in Peripherals.
Does that make sense?
