It's pure tabloid material on a level only British rags could produce, both the specific claim and the general idea. The specific claim usually has something to do with Zaida, mistress of Alfonso VI, their son Sancho, and something involving the prophet Mohammad. It's all nonsense. Contemporary Medieval sources aren't in accord regarding Zaida's background; the most reliable only assigns her a husband, not a parentage, so we can't even say for certain which ethnic group she came from.
More importantly, Zaida's only certainly-documented child was the aforementioned Sancho, who famously died without issue (or else Alfonso VI would not have left daughters as heirs, given he was preparing Sancho to succeed him). So we cannot say for certain that Zaida left any living descendants. She
might have been the mother of some of Alfonso's other illegitimate children, but so far as I'm aware no others are assigned to her in contemporary accounts.
Now, even
if this purported pedigree was correct (though as we've shown it wasn't), Queen Charlotte would hardly have been mixed race; she would have had one extremely distant ancestor (perhaps through a few different lines) that might have had a different ethnicity compared to most of the rest. This is incredibly common and not noteworthy. No one alive today sports an entirely 'pure' ancestry, and no one in the past did either. Race and ethnicity meant
very different things in the past, including 11th century Spain, and those terms are not directly comparable to the ones we use today.
The concept of race is more about phenotype than genetics anyway (there is more genetic diversity
within Africa than outside of it, unsurprisingly), and Queen Charlotte was plainly white as can be, was treated by society as white, and benefitted greatly from a system that exploited the Transatlantic Slave Trade for all it was worth. If that's not white privilgedge, I don't know what is.
Now, as for her
actual ancestry; she's a Mecklenburg, which means she descends (distantly, in the male line) from a group of North German Slavic pagans via
Niklot. She would also have descended from some Medieval Scandinavian royalty through
Euphemia of Sweden, and of course your usual hodge-podge of high German nobility that her more immediate ancestors would have married into. In any case, there would have been nothing special about her ancestry that was specific to her; she descended from the same people virtually any other German royal would have descended from. Certainly nothing which would qualify her to be mixed race in any meaningful way.