Light Goldstone boson and domain walls in the $K^0$-condensed phase of high density quark matter

It is pointed out that $K^0$ condensation in high density matter gives rise to an extremely light Goldstone boson whose mass comes entirely from weak interactions. This implies the existence of metastable non-topological domain walls with a long lifetime. We comment on the mass of the superfluid mode if baryon number is violated.