Name for a word that describes popular, short-lived words or expressions?
Quite a few writers have used fad word and fad expression just in the sense you mean. A fad is off course “something that people are interested in for only a short period of time” (Oxford Learner’s Dictionary). Here are some actual, self-explanatory examples (my boldface in all quotes):
Fad words that have come and gone in my lifetime include “groovy,” “awesome,” and “phat.” [Bruce Snoap, Gone: The Disappearance of Community in the Modern American Church, 2012.]
Perfectly good standard words are regularly replaced or supplemented merely for the additional spice, to flavour speech as thyme flavours soup. Most such words are fad words that disappear again almost immediately, especially among the young.
[Stephen R. Fischer, History of Language, 2004.]Various terms such as fad words or vogue words are applied to items that are popular for a relatively short time and are used more for their effect than for their precise meanings.
[Ian Hancock and Lorento Todd, International English Usage, 2005.]Although the phrase 23 Skidoo! (sometimes spelled 23 Skkidoo!) is now generally associated with the Roaring Twenties, it had in fact lost its popularity by the mid 1910s. During its heyday between 1900 and 1910, 23 Skidoo! was an expression to behold. Wentworth and Flexner credit it as having been “perhaps the first national fad expression and one of the most popular fad expressions to appear in the United States.”
[Tom Dalzell, Flappers 2 Rappers: American Youth Slang, 2012]
Yes, there is such a word, it's a vogue word
(linguistics) A neologism that gains sudden popularity but is forgotten after a relatively short time, such as wardrobe malfunction.
Not exactly a set-phrase or a slang-word, "a passing craze" seems to fit.
- "a short-lived popular fashion; a fad."
e.g.
"The hula-hoop was just a passing craze."
"I think the Pokémon GO is just a passing craze.
In addition, and more specific, "a fashionable phrase", "modish word".