  We present the simplest theory of perfect tunneling of an excitation in a
Bose-Einstein condensate (BEC) through an impurity potential with an arbitrary
shape in the low-momentum limit. That is for the transverse spin wave in the
ferromagnetic phase of a spin-1 BEC. This mode obeys a Schr\"odinger-type
equation; yet, effects of the potential on its transmission coefficient $T$ and
on its scattering cross section $sigma$ vanish in that limit. The order
parameter determines $T$, and the momentum $p$-dependence of $sigma$ exhibits a
Rayleigh scattering type ($sigma propto p^{4}$). These properties are common
between two types of Nambu-Goldstone modes: this spin wave and the Bogoliubov
mode.
