The phase oscillation of a charge density wave (CDW), known as a phason, is a massless Goldstone mode. However, when Coulomb interaction is poorly screened, the phason acquires mass. This massive phason was theorized decades years ago, yet its associated ultrafast local current oscillation had not been directly measured due to the absence of a suitable probe. Recently, THz emission from an unconventional CDW insulator (TaSe4)2I has been detected and suggests presence of the massive phason in this material. Motivated by this, we apply ultrafast scanning tunneling microscopy to (TaSe4)2I and directly resolve oscillating local current at 0.22 THz, whose temperature dependence match the predicted signatures of the massive phason in line with the THz emission study. Unexpectedly, we also discover a second mode at 0.11 THz with similar temperature dependence and comparable intensity. The robust 1:2 frequency locking across bias and temperature suggests that this 0.11 THz “daughter phason” arises from the splitting of the 0.22 THz massive phason into two massless phasons via parametric amplification. Furthermore, we observed that the emergence of the massive phason suppresses CDW amplitude oscillation, revealing an ultrafast dynamic competition between collective modes of different characters. Our studies reveal an unexplored mechanism for the generation and extinction of collective excitations and pave the way for a microscopic understanding of ultrafast phenomena in correlated materials.