  We numerically investigate slow dynamics of a binary mixture of ultrasoft
particles interacting with the generalized Hertzian potential. If the softness
parameter, $\alpha$, is small, the particles at high densities start
penetrating each other, form clusters, and eventually undergo the glass
transition. We find multiple cluster-glass phases characterized by different
number of particles per cluster, whose boundary lines are sharply separated by
the cluster size. Anomalous logarithmic slow relaxation of the density
correlation functions is observed in the vicinity of these glass-glass phase
boundaries, which hints the existence of the higher-order dynamical
singularities predicted by the mode-coupling theory. Deeply in the cluster
glass phases, it is found that the dynamics of a single particle is decoupled
from that of the collective fluctuations.
