  We reported the quasiparticle relaxation dynamics of an optimally doped
triclinic iron-based superconductor
(Ca$_{0.85}$La$_{0.15}$)$_{10}$(Pt$_3$As$_8$)(Fe$_2$As$_2$)$_5$ with bulk $T_c$
= 30 K using polarized ultrafast optical pump-probe spectroscopy. Our results
reveal anisotropic transient reflectivity induced by nematic fluctuations
develops below $T_{nem}$ $\approx$ 120 K and persists in the superconducting
states. Measurements under high pump fluence reveal three distinct, coherent
phonon modes at frequencies of 1.6, 3.5, and 4.7 THz, corresponding to
$A_{1g}(1)$, $E_g$, and $A_{1g}(2)$ modes, respectively. The high-frequency
$A_{1g}(2)$ mode corresponds to the $c$-axis polarized vibrations of FeAs
planes with a nominal electron-phonon coupling constant $\lambda _{A_{1g}(2)}$
$\approx$ 0.139 $\pm$ 0.02. Our findings suggest that the superconductivity and
nematic state are compatible but competitive at low temperatures, and the
$A_{1g}$ phonons play an important role in the formation of Cooper pairs in
(Ca$_{0.85}$La$_{0.15}$)$_{10}$(Pt$_3$As$_8$)(Fe$_2$As$_2$)$_5$.
