Quatrain 5

هر ذرّه که بر روی زمینی بوده است
خورشید‌رخی زهره‌جبینی بوده است
گرد از رخ نازنین به آزرم فشان
کان هم رخ خوب نازنینی بوده است

Dashti 37, p. 250

har zarre ke bar ru-ye zamini budast
khorshidrokhi zohrejabini budast
gard az rokh-e naazanin be aazarm feshaan
kaan ham rokh-e khub-e naazanini budast  

Every atom of dust ever on earth's face
had once been a radiant face, a Venus-brow.
brush the dust from your sweet face with respect;
that dust too had been someone, lovely and revered. 

Each particle of earth on ground you see—
A beauty proud like Venus once was she;
Ah, gently wipe the dust from Loveling's face—
That, too, was once a beauty fair and free.

Saidi, quatrain 86

Notable variants occur in the second and third lines of the Persian:
2nd line: پیش از من و تو تاج و نگینی بوده است  - "were before you and me, crown and ring-stone" (Forughi 50)
3rd line: instead of رخ نازنین  read رخ آستین rokh-e aastin, "sleeve-face" (Hedaayat 58)

Each mote on earth had once a royal birth,
Like sun a face, like Venus wits and worth;
So caress gently dust on Beloved's face,
It comes from lovers once so full of mirth.

Govinda Tirtha v. 22, from his "Clay and Cup" section
his final line reads کان هم رخ و زلف نازنینی بوده استو  "that too was a lovely face and tresses"

More of course than the plain fact of our existence and passing out of existence, this quatrain acknowledges the continuation of the beauty and splendor of those who have gone before.  We ourselves are endowed with the admirable qualities of our predecessors.   The speaker exhorts us to treat our continued existence with familial respect.
1. Every atom, speck of dust which has existed on earth's face 2. Has been a face like the sun, a brow like Venus.  These compound nouns are in the Indo-Iranian tradition.  They are exquisite.  Sir William Jones said: "One of the chief beauties of the Persian language is the frequent use of compound adjectives in the variety and elegance of which it surpasses not only the German and English but even the Greek." (See also article on Jones in the online Encyclopaedia Iranica.)  When I studied Greek at the university, in a class on Sophocles, we had a lecture on indoeuropean compounds, and we were taught the Sanskrit terminology to identify them.  Without this grounding we would not have had a clue how to translate certain compounds.  I generally use the Sanskrit (transcribed) in discussing these word formations in Khayyaam.  These two in the second line are bahuvrihi compounds, literally possessing/having "much rice" -- having a sun-face, having a Venus-brow.  But specifically, within the bahuvrihi category, they would be called "appositional descriptive compounds" -- having a face like the sun, having a brow like Venus (an example in Sanskrit is coincidentally chandrānana, चन्द्रानन,  "moon-faced").  On a grammatical note, the indefinite termination (ی, "i") is used here, "a ... face, a ... brow."
3. The dust from your lovely face brush it away with respect -- be aazarm, may mean "respectfully", it could be "gently" as Saidi and Tirtha have done, but it may be the esteem or respect owed to those close to the speaker, i.e., family.  I have taken naazanin as an adjective, but it could well be a noun, "sweetheart/beloved." This is the way Tirtha sees it and possibly Saidi, although I don't understand "loveling".  It's possible that Saidi means "the face of elegance, loveliness", which the speaker describes in the second line. 4. Since that (dust) also/too has been an admirable (and) a lovely face -- khub denotes an "admirable quality."

FitzGerald, according to Heron-Allen, did not use this quatrain for stanza XIX, 4th ed., below, but was inspired by Ouseley MS 43 (Heron-Allen, 33-35).  The Persian follows FitzGerald's rendition:

I sometimes think that never blows so red
The Rose as where some buried Caesar bled;
That every Hyacinth the Garden wears
Dropt in her lap from some once lovely Head.

هر جا که گلی و لاله‌زاری بودست
از سرخی خون شهریاری بودست
هر شاخ بنفشه کز زمین می‌روید
خالیست که بر رخ نگاری بودست

har jaa ke goli o laalezaari budast
az sorkhi-ye khun-e shahriyaari budast
har shaakh-e benafshe kaz zamin miruyad
khaalist ke bar rokh-e negaari budast

Where ruddy tulips grow and roses red,
Know that a mighty monarch's blood was shed;
And where the violet rears her purple tuft,
Be sure some black-moled girl doth rest her head.

Whinfield, 104