  Moir\'e superlattices are excellent platforms to realize strongly correlated
quantum phenomena, such as Mott insulation and superconductivity. In
particular, recent research has revealed stripe phases and generalized Wigner
crystals at fractional fillings of moir\'e superlattices. But these experiments
have not focused on the influence of electronic crystallization on the
excitonic and valleytronic properties of the superlattices. Here we report
excitonic and valleytronic signatures of correlated states at fractional
fillings in a WSe$_2$/WS$_2$ moir\'e superlattice. We observe reflection
spectral modulation of three intralayer moir\'e excitons at filling factors
$\nu$ = 1/3 and 2/3. We also observe luminescence spectral modulation of
interlayer trions at around a dozen fractional filling factors, including $\nu$
= -3/2, 1/4, 1/3, 2/5, 2/3, 6/7, 5/3. In addition, the valley polarization of
interlayer trions is noticeably suppressed at some fractional fillings. These
results demonstrate a new regime of light-matter interactions, in which
electron crystallization significantly modulates the absorption, emission, and
valley dynamics of the excitonic states in a moir\'e superlattice.
