  Using polarization-resolved electronic Raman scattering we study under-doped,
optimally-doped and over-doped Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ samples in the
normal and superconducting states. We show that low-energy nematic fluctuations
are universal for all studied doping range. In the superconducting state, we
observe two distinct superconducting pair breaking peaks corresponding to one
large and one small superconducting gaps. In addition, we detect a collective
mode below the superconducting transition in the B$_{2g}$ channel and determine
the evolution of its binding energy with doping. Possible scenarios are
proposed to explain the origin of the in-gap collective mode. In the
superconducting state of the under-doped regime, we detect a re-entrance
transition below which the spectral background changes and the collective mode
vanishes.
