  Recent NuSTAR and XMM-Newton observations of the molecular cloud around the
Arches stellar cluster demonstrate a dramatic change both in morphology and
intensity of its non-thermal X-ray emission, similar to that observed in many
molecular clouds of the Central Molecular Zone at the Galactic Center. These
variations trace the propagation of illuminating fronts, presumably induced by
past flaring activities of Sgr A$^{\star}$. In this paper we present results of
a long NuSTAR observation of the Arches complex in 2016, taken a year after the
previous XMM+NuSTAR observations which revealed a strong decline in the cloud
emission. The 2016 NuSTAR observation shows that both the non-thermal continuum
emission and the Fe K$_{\alpha}$ 6.4~keV line flux are consistent with the
level measured in 2015. No significant variation has been detected in both
spectral shape and Fe K$_{\alpha}$ equivalent width EW$_{\rm 6.4\ keV}$, which
may be interpreted as the intensity of the Arches non-thermal emission reaching
its stationary level. At the same time, the measured 2016 non-thermal flux is
not formally in disagreement with the declining trend observed in 2007-2015.
Thus, we cannot assess whether the non-thermal emission has reached a
stationary level in 2016, and new observations, separated by a longer time
period, are needed to draw stringent conclusions. Detailed spectral analysis of
three bright clumps of the Arches molecular cloud performed for the first time
showed different EW$_{\rm 6.4\ keV}$ and absorption. This is a strong hint that
the X-ray emission from the molecular cloud is a mix of two components with
different origins.
