It's potential energy (m_0 c^2), but in a way it's also kinetic because it is a moving wave, it's just that it's a standing wave so it's as though it's reflecti

It's potential energy (m_0 c^2), but in a way it's also kinetic because it is a moving wave, it's just that it's a standing wave so it's as though it's reflecting, but being a standing wave causes that part of the particle's bundle of energy to manifest as potential energy.