We investigate the magnetic order in single crystalline EuCd2Sb2 thin films using a combined theoretical and experimental approach. Resonant elastic x-ray scattering experiments reveal a sharp magnetic peak at q=(0,0,12) below TN=7.2 K, indicative of interlayer antiferromagnetic ordering. Additionally, we observe a weak diffuse magnetic signal centered at q=(0,0,1) that persists above TN, up to TC∼11 K. Our Monte Carlo simulations of a classical spin model approximation of the Eu magnetic sublattice demonstrate that the diffuse signal can arise from ferromagnetic coupling in the top few layers due to surface oxidation. On the other hand, the bulk of the sample exhibits antiferromagnetic coupling between layers. Finally, our fit of the model parameters to the magnetic ordering temperatures shed light on the exchange couplings that are key in stabilizing the observed composite magnetic order.