  The study of exciton-polarons has offered profound insights into the
many-body interactions between bosonic excitations and their immersed Fermi sea
within layered heterostructures. However, little is known about the properties
of exciton polarons with interlayer interactions. Here through magneto-optical
reflectance contrast measurements, we experimentally investigate interlayer
Fermi polarons for 2s excitons in WSe$_2$/graphene heterostructures, where the
excited exciton states (2s) in the WSe$_2$ layer are dressed by free charge
carriers of the adjacent graphene layer in the Landau quantization regime.
First, such a system enables an optical detection of integer and fractional
quantum Hall states (e.g. $\nu=\pm1/3$, $\pm$2/3) of monolayer graphene.
Furthermore, we observe that the 2s state evolves into two distinct branches,
denoted as attractive and repulsive polarons, when graphene is doped out of the
incompressible quantum Hall gaps. Our work paves the way for the understanding
of the excited composite quasiparticles and Bose-Fermi mixtures.
