  The recent survey of the core-collapse supernova remnant Cassiopeia A (CasA)
with the MIRI instrument on board the James Webb Space Telescope (JWST)
revealed a large structure in the interior region, referred to as the "Green
Monster". Although its location suggests that it is an ejecta structure, the
infrared properties of the "Green Monster" hint at a circumstellar medium (CSM)
origin. In this companion paper to the JWST Cas A paper, we investigate the
filamentary X-ray structures associated with the "Green Monster" using Chandra
X-ray Observatory data. We extracted spectra along the "Green Monster" as well
as from shocked CSM regions. Both the extracted spectra and a principal
component analysis show that the "Green Monster" emission properties are
similar to those of the shocked CSM. The spectra are well-fit by a model
consisting of a combination of a non-equilibrium-ionization model and a
power-law component, modified by Galactic absorption. All the "Green Monster"
spectra show a blueshift corresponding to a radial velocity of around -2300
km/s, suggesting that the structure is on the near side of Cas A. The
ionization age is around $n_{e}t = 1.5 \times 10^{11}$ cm$^{-3}s. This
translates into a pre-shock density of ~12 cm$^{-3}$, higher than previous
estimates of the unshocked CSM. The relatively high net and relatively low
radial velocity suggest that this structure has a relatively high density
compared to other shocked CSM plasma. This analysis provides yet another piece
of evidence that the CSM around Cas A's progenitor was not that of a smooth
steady wind profile.
