His home is surrounded by a powerful elemental enchantment that enables him to grow land plants and even keep a pet Cat, whilst not interfering with any spells that may be affecting his guests - such as magical gills (or lungs, in the case of his aquatic neighbours). The book illustration depicts him barefoot; this may have to do with his eccentricity and/or the fact that he has no need for footwear due to living at the sea bottom.
He maintains communication of sorts with people on the surface, who place magically protected letters in bottles and let them fall to the ocean floor; and is a friendly, if brisk and no-nonsense, individual. At one time, Greylock aided a sailor in getting revenge on the pirates who threw him overboard to drown, but unwittingly did so over an ancient Atlantean pentagram that bestowed gills upon him, thereby allowing him his chance at revenge.
This service was done willingly, with the wizard informing the sailor of the power of the Black Pearls for free. However, in exchange for a Silver Pearl, he also offered the sailor a choice of one of three magical potions: a Potion of Fortune, a Potion of Healing and a Potion of Vaporous Essence.