The detection of high-energy astrophysical neutrinos by IceCube implies production of high-energy photons in the same sources. I present a new technique that uses this photon flux to search for a diffuse axion-like particle (ALP) background. These photons may convert into ALPs in the sources' magnetic fields, and will travel as ALPs through extragalactic space. Back-conversion in the Milky Way’s magnetic field leads to a diffuse anisotropic high-energy photon flux that existing and upcoming gamma-ray detectors, like HAWC, CTA, and LHAASO can detect. I show that LHAASO will be realistically sensitive to couplings above $10^{-11}~{\rm GeV}^{-1}$ and masses up to $3 \times 10^{-6}$ eV in ten years, thereby probing even viable dark matter parameter space.
