We report the results of measurements of the low-temperature specific heat C_p(T) of annealed samples of decagonal Al_69.8Mn_18.1Pd_12.1. A comparison with similar data, previously obtained for decagonal Al-Cu-Co quasicrystals, indicate a substantial excess specific heat C_ex(T) in the Al-Mn-Pd material, most likely due to magnetic degrees of freedom. The temperature dependence of C_ex(T) suggests a spin freezing phenomenon at T_f=12 K, which is confirmed by the temperature dependence of the third-order magnetic susceptibility χ₃(T) and the time dependence of the remanent magnetization well below T_f.