*

The transient nematicity, coherent phonon, and quasiparticle dynamics were explored for FeSe_0.8Te_0.2 superconducting epitaxial films. A nematicity in coherent phonon dynamics was resolved by applying an ultrafast pump-probe polarization optical technique. This dynamic is inherent for FeSe_0.8Te_0.2 near and below Tc, and disappears as soon as the material switches to a normal state. The superconducting state persists during electron-electron thermalization within several hundreds of femtoseconds and further decays into a normal state during ~1 ps via electron-phonon scattering.  By applying a two-temperature model, we modeled processes of electron-electron and electron-phonon scattering, and calculated photoinduced stress in the film. Derived coherent phonon dynamics show a dependence of electron-phonon coupling on the stress in the film. To resolve structural changes in optical signal, we applied the 3D angle-resolved light scattering method, including the diffraction conoscopy technique. Here we retrieved surface autocorrelation, surface roughness spectral power density, and lattice distortion in the region of superconducting transition. These results show good agreement with observed photoinduced transient dynamics at corresponding temperatures. Analysis is performed with additional DFT calculations of the phonon and electron density of states. Our findings hint at novel underlying mechanisms governing the photoinduced relaxation dynamics in FeSe_1-xTe_x superconductors.