  Nambu-Goldstone (NG) modes for 0-form and higher-form symmetries can become
unstable in the presence of background fields. Examples include the instability
of a photon with a time-dependent axion background or with a chirality
imbalance, known as the chiral plasma instability, and the instability of a
dynamical axion with a background electric field. We show that all these
phenomena can be universally described by a symmetry algebra for 0-form and
higher-form symmetries. We prove a counting rule for the number of unstable NG
modes in terms of correlation functions of broken symmetry generators. Based on
our unified description, we further give a simple new example where one of the
NG modes associated with the spontaneous 0-form symmetry breaking $U(1) \times
U(1) \to \{1\}$ becomes unstable.
