Near the end of Telemachus, on old man pops to the surface of the water and clambers up the rocks, water running off of his body and bathing trunks. Glancing at Stephen and Haines, Mulligan crosses himself, which is our cue to infer that the swimmer is a priest. The suspicion is confirmed later when the narrative mentions "The priest's grey nimbus in a niche where he dressed discreetly." The nimbus seems to refer to a "garland of grey hair" seen earlier on the old man, and the niche to a gap between rocks on the shore where he can modestly remove his swimsuit.
"Nimbus" (Latin for cloud) is a plausibly descriptive detail for a small bit of hair seen from a distance, in the gloom of the rocky “niche” in which the priest modestly dresses. Both words have religious connotations, however. In the works of antiquity, gods often concealed themselves in dark clouds or haloes of light when they appeared to men and women on earth. Virgil’s Aeneid frequently shows the gods hiding themselves from the men whom they favor in clouds. "Niche," in Catholic usage, refers to a shallow architectural recess (e.g., a semicircular space in the wall of a medieval cathedral) designed to hold a statue (e.g., of a saint). So these two words work together to suggest some kind recessed, concealed divinity, consistent perhaps with Stephen's thought that the old milkwoman is "maybe a messenger," some "lowly form of an immortal."
In Proteus, Stephen thinks again of the old man's "garland of grey hair," associating it now with the supposedly bald head of Joachim of Fiore: "Get down, bald poll!" Both men make him think of the man he might have become had he answered the calling of priesthood.