By optically exciting the guided acoustic waves in an optical fiber, forward stimulated Brillouin scattering (FSBS) enables the identification of external substances by measuring the acoustic impedance while introducing no optical power leakage. In most experimental demonstrations, uncoated silica optical fibers are adopted to achieve a better sensing performance. However, fragility makes such bare fibers not ideal for practical applications. In this work, environmental acoustic impedance measurement based on FSBS is demonstrated using an aluminum-coated optical fiber, exploring the stable material properties of aluminum and its role as a low-loss medium for acoustic wave propagation. From the detected FSBS-induced temporal waveforms, acoustic reflections at the silica-aluminum and the aluminum-environment interfaces are identified and processed for acoustic impedance measurement of the surrounding liquid. The impedance values calculated from each set of measurement data maintain a deviation of no more than 5% from the reference values, concurrently achieving a substantial reduction in measurement time.