Witchcraft is simply another way of saying magic. It is a little known fact that merfolk practice magic, though hardly surprising when you think about it. After all, their sentience is essentially no different from that of humans or elves, and therefore they have the same capacity for magic. Unlike elves, who chose not to use the human term Sorreters to denote magic-users among their race in a conscious effort to distance themselves from the human race, merfolk never had any particular resentment towards humanity. However, by chance they did choose to use a different term to refer to magicians among their race, but the decision to do so was fairly arbitrary. Mermaids who practice magic are referred to as "witches," while mermen who practice magic are referred to as "warlocks." Unlike both human and elven magic-users, there is no rank classification system other than "apprentice" and "master." However, unlike the majority of human and elven society (not merely in regards to magic, but any type of skill or trade), the term "master" among merfolk does not necessarily mean "teacher," but simply one who has reached a certain skill level. Often this means they have completed their training, and are therefore no longer an apprentice; however, this does not preclude them from furthering their studies and increasing their skill. It is therefore often impossible to estimate how skilled a witch or warlock may in fact be, and is therefore considered both polite and wise to treat all masters with respect. (Of course, among the magical community, many may be known for particular skill, and in an informal sense some masters may refer to others as "grand masters," though this is not an official rank.)
It should also be noted that "witchcraft" is a term taken from Earth, where it is often considered to be a negative or "black" form of magic, possibly associated with the worship of Lucifer. This was already an archaic attitude when The Land was created, and by that time the term "witch" had largely been supplanted by "Wiccan." There were, even then, people who considered the practice of wicca to be inherently evil, though even many who know little or nothing about the practice had by then come to realize this was not necessarily the case. Certainly it is not the case with witches and warlocks on the Land, or at least it is no more likely to be the case than it is with human Sorreters or elven sorcerers. That is, the spirits from whom they learn are no more likely to be followers of Lucifer than of God, and there is no real difference between the nature of the magic employed in witchcraft, Sorretry, and sorcery.