Coral cores taken from Great Barrier Reef massive Porites sp. were assessed for bioerosion by the brown demosponge Cliona orientalis Thiele, 1900, but also yielded evidence for microbial bioerosion that was partly simultaneously active with the sponge bioerosion. The most common microborer traces throughout were Ichnoreticulina elegans (Radtke, 1991) produced by the chlorophyte alga Ostreobium quekettii Bornet and Flahault, 1889 and Scolecia serrata Radtke, 1991, likely a bacterial trace. Additional, less common traces belonging to the ichnogenera Saccomorpha and Orthogonum were attributed to fungal tracemakers. Especially I. elegans was found partly colonizing the so-called sponge scars, new surfaces formed by the bioeroding sponge, indicating that the alga was still actively bioeroding after infestation by the sponge. In addition, a very small but abundant trace was discovered directly associated with the sponge’s etching scars. It is a yet-undescribed trace apparently of either fungal or bacterial origin. We found evidence that this and some other microbial euendoliths can at least temporarily co-exist with the clionaid sponge, both eroding simultaneously, and potentially even being engaged in a yet-unspecified symbiotic relationship. Observations on such endolithic borer relationships across different taxa are very rare. Further studies are needed to elucidate the roles of different bioeroders as well as their potential interactions.