We report elastic light scattering measurements of the surface morphology of strained InxGa1-xAs on GaAs, grown by molecular beam epitaxy at different growth temperatures and In contents. During strain relaxation through formation of interfacial misfit dislocations, the surface of the film roughens in response to inhomogeneous surface strains produced by the interfacial misfit dislocations. The time dependence of this roughening is modeled by an Edwards-Wilkinson equation in which the deposition flux noise is neglected and the inhomogeneous surface stress is the only driving term.