Jim writes:

Altitude Bugs
Altitude Alert Indicator marks critical altitude levels so they can be
maintained by quick visual reference. In the track are three tabs that
move simultaneously: green tab points to 500' above DH or MDA, yellow
tab points from 200' to 100' above minimums and red tab is placed at DH
or MDA. An added safety margin for approaches, it will also help sharpen
your skills. Altitude Alert Indicator 2 7/8" dia.

Mystery solved! Thanks! Odd that Reality XP includes this since it
apparently isn't part of the instrument itself. In any case, I found out how
to turn it off.