Spin-degenerate antiferromagnets can host hidden unconventional magnetism in their lattice degrees of freedom. We show that coherent phonons activate this magnetism by removing the spin-layer operations that enforce equilibrium band degeneracy without changing the collinear Néel order. The frequency and polarization of a pump electric field select a resonant Γ-point optical phonon and, within a doublet, its coordinate direction. This choice fixes the residual spin-layer symmetry. In monolayer MnPSe₃, an A₂ᵤ mode produces i-wave splitting odd in the mass-weighted phonon coordinate Q, reversing sign under Q→-Q, whereas two orthogonal directions of the same doubly degenerate Eᵤ doublet produce d- and s-wave splitting at a common resonance. Rotating the in-plane pump field E_∥ therefore programs both the spin-splitting texture and the thermoelectric spin current, continuously tuning the response between transverse pure-spin and longitudinal spin-polarized currents. A complete classification of two-dimensional collinear spin layer groups identifies the Γ-point coordinates that remove the degeneracy-enforcing operations and activate such unconventional magnetism.