  We discuss the effective metric experienced by the Nambu-Goldstone mode
propagating in the broken symmetry spin-superfluid state of coherent precession
of magnetization. This collective mode represents the phonon in the RF driven
or pulsed out-of-equilibrium Bose-Einstein condensate (BEC) of optical magnons.
We derive the effective BEC free energy and consider the phonon spectrum when
the spin superfluid BEC is formed in the anisotropic polar phase of superfluid
3He, experimentally observed in uniaxial aerogel 3He-samples. The coherent
precession of magnetization experiences an instability at a critical value of
the tilting angle of external magnetic field with respect to the anisotropy
axis. From the action of quadratic deviations around equilibrium, this
instability is interpreted as a Minkowski-to-Euclidean signature change of the
effective phonon metric. We also note the similarity between the magnon BEC in
the unstable region and an effective vacuum scalar "ghost" condensate.
