  RZ2109 is the first of several extragalactic globular clusters shown to host
an ultraluminous X-ray source. RZ2109 is particularly notable because optical
spectroscopy shows it has broad, luminous [OIII] 4959,5007 emission, while also
having no detectable hydrogen emission. The X-ray and optical characteristics
of the source in RZ2109 make it a good candidate for being a stellar mass black
hole accreting from a white dwarf donor (i.e. an ultracompact black hole X-ray
binary). In this paper we present optical spectroscopic monitoring of the
[OIII]5007 emission line from 2007 to 2018. We find that the flux of the
emission line is significantly lower in recent observations from 2016-2018 than
it was in earlier observations in 2007-2011. We also explore the behaviour of
the emission line shape over time. Both the core and the wings of the emission
line decline over time, with some evidence that the core declines more rapidly
than the wings. However, the most recent observations (in 2019) unexpectedly
show the emission line core re-brightening.
