  After numerous astronomical and experimental searches, the precise particle
nature of dark matter is still unknown. The standard Weakly Interacting Massive
Particle(WIMP) dark matter, despite successfully explaining the large-scale
features of the universe, has long-standing small-scale issues. The spectral
distortion in the Cosmic Microwave Background(CMB) caused by Silk damping in
the pre-recombination era allows one to access information on a range of small
scales $0.3 \, {\rm Mpc} < k < 10^4 \, \rm Mpc^{-1}$, whose dynamics can be
precisely described using linear theory. In this paper, we investigate the
possibility of using the Silk damping induced CMB spectral distortion as a
probe of the small-scale power. We consider four suggested alternative dark
matter candidates---Warm Dark Matter (WDM), Late Forming Dark Matter, Ultra
Light Axion dark matter and Charged Decaying Dark Matter; the matter power in
all these models deviate significantly from the $\Lambda$CDM model at small
scales. We compute the spectral distortion of CMB for these alternative models
and compare our results with the $\Lambda$CDM model. We show that the main
impact of alternative models is to alter the sub-horizon evolution of the
Newtonian potential which affects the late-time behaviour of spectral
distortion of CMB. The $y$-parameter diminishes by a few percent as compared to
the $\Lambda$CDM model for a range of parameters of these models: LFDM for
formation redshift $z_f = 10^5$ (7\%); WDM for mass $m_{\rm wdm} = 1 \, \rm
keV$ (2\%); CHDM for decay redshift $z_{\rm decay} = 10^5$ (5\%); ULA for mass
$m_a = 10^{-24} \, \rm eV$ (3\%). We also briefly discuss the detectability of
this deviation in light of the upcoming CMB experiment PIXIE, which might have
the sensitivity to detect this signal from the pre-recombination phase.
