How would one say "fading away" in Greek and Latin?

Question

In music, the Italian term morendo means "fading or dying away". This fairly transparently comes from morior. But morior to me suggests the moment of death, rather than a gradual decline, and is also a fairly morbid word.

If I wanted an adjective (or participle) for "fading away" or "in decline", are there better alternatives to moriēns? I'm interested in both Greek and Latin.

ETA: Example sentences:

The Roman empire was in decline for a significant period before its actual fall.
We sat by the ocean at dusk, watching the sunlight fading away.
He would have preferred a quick death rather than fading away like this.

Answer 1

LATIN

senescens (from senescere - to grow old, weak, feeble; to waste away; to wane, fall off, decline)

Can be used literally, naturally, of someone growing weak with old age, but also figuratively:

et Hannibalem ipsum iam et fama senescere et viribus

and Hannibal himself now was in decline, in both reputation and strength

Livy, History of Rome, 29.3

Can also be used more figuratively:

senescet amor

love will fade away

Ovid, Ars Amatoria, 3.594

GREEK

μαραινόμενος (from μαραίνομαι - to die away; to waste away; to slowly go out; to abate)

Can be used of people:

καὶ τὸ σῶμα … οὐκ ἐμαραίνετο

and the body … was not wasted away

Thucydides, Peloponnesian War, 2.49

Can also be used to describe inanimate things:

φλὸξ ἐμαράνθη

[the] fire died down

Homer, Iliad, 9.213

μαραινομένου τοῦ πνεύματος

with the wind dying away

Plutarch, Pyrrhus, 15

Answer 2

I'm not sure that I agree with your premise: morior, 'die,' gives moriendum, 'a dying.' Could the Italian have mutated, through popular association, from morando?
Moror comes in three forms from three different roots; mŏs (usage) and mōrus (silly) are dead ends; but what about mora, delay?

morans, morantes, lingering.

Another musical term

Con sordini 'with a mute'

suggest sordeo. The problem is sorditus had connotations in classical Latin of meanness, grime, decay.

May I suggest cadens as the most promising. There is the musical association with 'cadence,' and a quotation from Horace;

Cadentia verba. 'words dying away;'