  In the modular representation theory of finite unitary groups when the
characteristic $\ell$ of the ground field is a unitary prime, the
$\hat{\mathfrak{sl}}_e$-crystal on level $2$ Fock spaces graphically describes
the Harish-Chandra branching of unipotent representations restricted to the
tower of unitary groups. However, how to determine the cuspidal support of an
arbitrary unipotent representation has remained an open question. We show that
for $\ell$ sufficiently large, the $\mathfrak{sl}_\infty$-crystal on the same
level $2$ Fock spaces provides the remaining piece of the puzzle for the full
Harish-Chandra branching rule.
