  In this paper, we study the nonlinear Choquard equation \begin{eqnarray*}
\Delta^{2}u-\Delta u+(1+\lambda a(x))u=(R_{\alpha}\ast|u|^{p})|u|^{p-2}u
\end{eqnarray*} on a Cayley graph of a discrete group of polynomial growth with
the homogeneous dimension $N\geq 2$, where
$\alpha\in(0,N),\,p>\frac{N+\alpha}{N},\,\lambda$ is a positive parameter and
$R_\alpha$ stands for the Green's function of the discrete fractional
Laplacian, which has same asymptotics as the Riesz potential. Under some
assumptions on $a(x)$, we establish the existence and asymptotic behavior of
ground state solutions for the nonlinear Choquard equation by the method of
Nehari manifold.
