The Spanish mancha (“spot” or “stain”) comes from the Latin word for the same, macula.
From the Latin macula, we get the English… immaculate — which literally means (knowing the negation prefix of im-) “without a stain.” So the immaculate conception truly was perfect!
How this sound changed was interesting: often Latin words with a ct- or cl- or other hard letters after a c- sound turn into a suave ch in Spanish. For a distant example, see duct and ducha, or nocturnal and noche. (The ct- is much more common than the cl-, but they’re cousins!) Thus, we can see the m-ch of mancha mapping to the (im-)m-cl of immaculate.