  Monoclinic semi-metallic Mo$_8$O$_{23}$ belongs to a multifunctional series
of compounds showing multiple ordering phenomena that have not achieved much
attention till now. Previous X-rays studies of this compound have revealed an
incommensurate ordering transition at $T_{\mathrm{IC}}\sim350$ K, followed by a
structural transition to commensurate order at $T_{\mathrm{IC-C}}=285$ K. In
addition, an enigmatic resistance maximum is observed at
$T_{\mathrm{el}}\sim150$ K, whose origin has so far proved elusive. Aiming to
disentangle these multiple orders we use the polarized transient optical
spectroscopy supplemented by Raman spectroscopy to study the electronic
relaxation dynamics and lattice vibrational modes in Mo$_8$O$_{23}$ single
crystals. Remarkably, both the coherent vibrational mode response and single
particle response display extrema of damping/relaxation times close to
$T_{\mathrm{el}}$ with the concurrent appearance of new coherent vibrational
modes and a characteristic polarization asymmetry which saturates below
$T_{\mathrm{el}}$. The single-particle relaxation data analysis shows the
appearance of a temperature-independent gap in the electronic excitation
spectrum below $T_{\mathrm{IC}}$ and additional temperature-dependent gap
opening near $T_{\mathrm{el}}$. Concurrently, a low frequency vibrational mode
shows anomalous softening around $T_{\mathrm{m}}\sim200$ K, far below
$T_{\mathrm{IC-C}}$ and $T_{\mathrm{IC}}$. The observations are interpreted in
terms of the appearance of a hidden gapped state below $T_{\mathrm{el}}$ that
has so far eluded detection by structural analyses.
