  We present new observations of MRG-M2129, a quiescent galaxy at z = 2.15 with
the Atacama Large Millimeter/submillimeter Array (ALMA). With the combination
of the gravitational lensing effect by the foreground cluster and the angular
resolution provided by ALMA, our data reveal 1.2 mm continuum emission at
$\sim130$ pc angular resolution. Compact dust continuum is detected at 7.9
$\sigma$ in the target but displaced from its stellar peak position by $62 \pm
38$ mas, or $\sim169 \pm 105$ pc in the source plane. We find considerably high
dust-to-stellar mass ratio, $4 \times 10^{-4}$. From non-detection of the [C i]
3P2 -> 3P1 line, we then derive $3 \sigma$ upper limits on the molecular
gas-to-dust mass ratio $\delta_\mathrm{GDR} < 60$ and the molecular
gas-to-stellar mass ratio fH2 < 2.3%. The derived $\delta_\mathrm{GDR}$ is
$>2\times$ smaller than the typical value assumed for quiescent galaxies in the
literature. Our study supports that there exists a broad range of
$\delta_\mathrm{GDR}$ and urges submillimeter follow-up observations of
quenching/recently quenched galaxies at similar redshifts. Based on the derived
low $\delta_\mathrm{GDR}$ and other observed dust properties, we argue that the
central black hole is still active and regulates star formation in the system.
Our study exhibits a rare case of a gravitationally lensed type 2 QSO harbored
by a quiescent galaxy.
