Transient
Etymology
From Latin transiens, the present participle of transire, "to go across" (trans, "across," + ire, "to go"). The literal image is of something merely passing through rather than settling in. That sense survives directly in the noun — a transient is a person just passing through a town — and in the adjective’s meaning of "short-lived."
adjective · /TRAN-zee-uhnt/
Lasting only a short time before passing away.
What it names
The right word for anything that is present now but plainly on its way out — a mood, a resident, a state of matter — with the emphasis on its passing through rather than staying.
Example
The gallery specialized in transient art — sand mandalas and ice sculptures made to be admired and then lost.
Related idea
Evanescent stresses the beautiful act of vanishing; transient is more neutral and factual, simply noting that a thing will not last.