We describe powder and single-crystal inelastic neutron scattering experiments on a spinel-type antiferromagnet GeCo2O4, represented by an effective total angular momentum J eff = 1/2. Several types of non-dispersive short-range magnetic excitations were discovered. The scattering intensity maps in Q space are well reproduced by dynamical structure factor analyses using molecular model Hamiltonians. The results of analyses strongly suggest that the molecular excitations below TN arise from a hidden molecular-singlet ground state, in which ferromagnetic subunits are antiferromagnetically coupled. The quasielastic excitations above TN are interpreted as its precursor. A combination of frustration and J eff = 1/2 might induce these quantum phenomena.