Excitons provide a sensitive optical probe of electronic correlations in nearby two-dimensional materials, yet their coupling to intrinsic flat-band Mott systems remains largely unexplored. Here we combine gate-tunable optical spectroscopy with first-principles calculations to study monolayer WSe₂ in direct contact with the van der Waals Mott insulator Nb₃Cl₈. The gate evolution of WSe₂ excitonic resonances reveals signatures of a correlation-reconstructed Mott gap in Nb₃Cl₈ that is absent from the single-particle band picture. In the electron-doped regime, the WSe₂ 2s Rydberg exciton undergoes a multistage evolution and develops into interlayer attractive and repulsive polaron branches, showing that a Rydberg exciton can be dressed by strongly correlated flat-band electrons in an adjacent Mott layer. Under an out-of-plane magnetic field, spin-polarized Nb₃Cl₈ states further induce valley-selective exciton coupling, producing a strongly enhanced circular polarization of the WSe₂ exciton emission. These results extend exciton-based sensing and exciton-polaron physics to intrinsic flat-band Mott materials, providing an optical route to probe and engineer correlation-driven interfacial quasiparticles.