Let $G$ be the simple group ${\rm PSU}(2,2^{2^n})$, $n>0$. For any subgroup $H$ of $G$, we compute the Möbius function $μ_L(H,G)$ of $H$ in the subgroup lattice $L$ of $G$, and the Möbius function $μ_{\bar L}([H],[G])$ of $[H]$ in the poset $\bar L$ of conjugacy classes of subgroups of $G$. For any prime $p$, we provide the Euler characteristic of the order complex of the poset of $p$-subgroups of $G$.