Cosmic microwave background (CMB) photons can convert into axions as they cross the halo magnetic fields of non-linear structure. Resonant conversion occurs when the axion mass matches the photon plasma mass, induced by the ionized gas within halos, leading to a frequency-dependent transition probability. Therefore, the axion induces a frequency-dependent anisotropic screening of CMB photons which imprints secondary anisotropies in the observed CMB temperature and polarization fields. I will discuss how to compute this axion signal and its expected correlation functions, including cross-correlations between CMB and Large Scale Structure. I will then show that, using these observables, future CMB and galaxy surveys could provide the best probe of light axions that couple to photons.