  Ferroelectricity has been recently discovered in stacked or twisted van der
Waals (vdW) moir\'e systems. The versatility of producing a large array of
size-tunable domains and integration with diverse functional materials make
them an enticing platform for developing multifunctional devices. Here, we show
that ferroelectric polar domains formed in a twisted hexagonal boron nitride
(t-hBN) substrate can modulate light emission from an adjacent semiconductor
monolayer. The abrupt change in electrostatic potential across the domains
produces an in-plane electric field (E-field) and leads to a remarkably large
exciton Stark shift in the adjacent MoSe$_2$ monolayer, previously only
observable in \textit{p-n} junctions created by the advanced e-beam lithography
tools. Both the spectrum and spatial pattern of the light emission from the
monolayer are periodically modulated by the remote moir\'e potential imposed by
the t-hBN substrate. We further observe a characteristic hysteresis behavior in
the light emission as an electric gate erases and restores the domains. Our
findings chart an exciting pathway for integrating nanometer-scale moir\'e
ferroelectric domains with various optically active functional layers, paving
the way for advanced nanophotonic applications.
