Merfolk communication

A unique type of communication used by merfolk, sometimes referred to more simply as Mermish. Resembling telepathy, merfolk communicate in memory fragments (or, sometimes, more complete memories), emotions, and feelings. Though it uses magic, it uses magic in an innate way, and it's almost impossible to replicate this method of communication for members of other species.
Mermish can be spoken one-to-one or broadcast over a wider reach. In the latter's case, the distance and clarity over said reach can be controlled, with outer bounds depending on the individual, often linked to their magical strength. For those outside of the reach, or not participating in one-on-one communication, it's impossible to hear or intercept merfolk communication: it takes place entirely within one's mind.

Sentence Structure

Mermish is spoken without words, and without much structure. How memories, emotions, and feelings are used will vary depending on what is being communicated. A merperson trying to ask another to hand them something may communicate it by offering a memory of them holding said object, overlaid with a sense of desire, then a questioning feeling. Communicating one's anger is simply a case of letting the other sense said anger, sometimes combined with a memory fragment of the cause.
Sometimes feelings and emotions may be overlaid with memory fragments, sometimes they overlap only partially or come after (or before). Sometimes feelings and emotions are offered separately, sometimes they're felt simultaneously. There is no set structure to the language, just like there is no set structure to, well, the mind!

Common Phrases

Because Mermish is a telepathic language without words, there aren't many 'phrases', so to say, which may be common usage. Some common phrases do have a common method of communicating them, however, a base-line which may be adapted to the specific sentiment carried.
Greeting someone, saying hello, is typically communicated with a sense of greeting, a feeling of welcoming someone. Saying farewell is often communicated with a memory of someone (possibly that person, sometimes more generically) swimming away, often combined with how that makes the person feel. Swear words don't really exist, but the same sentiment is usually communicated by a short but vicious outburst of anger or frustration.