  Recent neutron scattering experiments suggested that frustrated magnetic
interactions give rise to antiferromagnetic spiral and fractional skyrmion
lattice phases in MnSc$_2$S$_4$. Here, to trace the signatures of these
modulated phases, we studied the spin excitations of MnSc$_2$S$_4$ by THz
spectroscopy at 300 mK up to 12 T. We found a single magnetic resonance with
linearly increasing frequency in field. The corresponding $g$-factor of
Mn$^{2+}$ ions $g$ = 1.96, and the absence of other resonances imply very weak
anisotropies and negligible contribution of higher harmonics to the spiral
state. The significant difference between the dc magnetic susceptibility and
the lowest-frequency ac susceptibility in our experiment implies the existence
of mode(s) below 100 GHz.
