The discovery of the hydrodynamic electron liquid (HEL) in graphene [D. Bandurin \emph{et al.}, Science {\bf 351}, 1055 (2016) and J. Crossno 1058 (2016)] has marked birth solid-state HEL which can be probed near room temperature a table-top setup. Here we examine terahertz (THz) magneto-optical (MO) properties HEL. Considering case where magnetic length $l_B=\sqrt{\hbar/eB}$ is comparable to m...