Great question. I have looked through every single relevant primary and secondary source that I have in my library (that's a lot!) and while many mention the name "White/Pale Death of the Saracens" being attached to Nikephoros II Phokas from his successful campaigns against the Muslims, I was unable to find any explanations of the name.

I do not believe the name refers to a physical feature, since Nikephoros is said to have had darker skin according to both Leo the Deacon and Liutprand of Cremona. However, I would surmise that the name's meaning has a two-fold purpose. First, to recall the draining of blood from the face due to fear, since the Saracens are said to have been utterly terrified of Nikephoros on the battlefield (as evidenced by their nickname given: "the Hammer" is what they called him). Second, Nikephoros by all accounts was an extremely pure and pious man, regarded as an ascetic by many. "White/Pale" could also refer to a notion of rightly-guided purity (White is a common symbol of purity and righteousness in the Bible, ex: "For the wedding of the Lamb has come, and his bride has made herself ready. Fine linen, bright and clean, was given her to wear.”- Revelation 19:7-8) . In essence, Nikephoros is seen as a manifestation of pure and divine power, sent to annihilate the enemies of Christendom on the battlefield. Further credence is given by the fact that Nikephoros supposedly bore with him a piece of cloth of John the Baptist, recovered from his conquests, which draws parallels with the 'ascetic' motif of Nikephoros' life.

In regards to your second question regarding the Saracens: let us recall the tale of Abraham in the Book of Genesis. The relevant part of the story begins with Abram (an old man) being told by God in a dream that he is to be the father of God's nation. However, he has trouble believing it because both he and his wife Sarai are of advanced age and incapable of having children. In order to fulfill the prophecy, Sarai tells Abram to conceive a child with her young chambermaid Hagar - and this child's name was Ishmael (from which the Muslims were believed to have been descended). However, some years later, God contacts Abram again and bestows upon him the name Abraham and again tells him that he is to be the father of a nation. He reveals to Abraham a new covenant which Abraham follows, and having followed God's wishes, Abraham and Sarah finally conceive a child together named Isaac, who is the father of Jacob, patriarch of the Israelites, and therefore a progenitor of Christianity.

Medieval authors (especially the Byzantines) used the terms Agarenes (Ἀγαρηνοί) and Saracens (Σαρακηνοί) to describe their Islamic adversaries, creating an additional "divinely reported" (that is, from the Bible) genealogical gulf to further differentiate themselves from their foes. The term 'Agarene' (used more often in earlier Byzantine sources) derives from the name of Abraham's first wife, Hagar. The term 'Saracen' derives from a name given by Late Roman authors to the people of a region of Arabia, the Sarakeni. However, interpretations by medieval historians attempting to reconcile the similarity with Sarah's name believed that Saracen was a term applied by the Muslims to themselves to cover up the fact that their progenitor Ishmael was not born of the true wife of Abraham (and in some traditions was in fact a concubine). A scholarly note in the Byzantine Suda encyclopedia further elaborates by saying:

"Originally and literally the denizens of Saraka, a territory in Arabia (present-day Yemen). However, in late-antique and early-medieval usage the application of the term progessively broadened out, first to include non-Arab people(s) of the region and ultimately to become a generalised synonym for Arabs/Muslims."