What is the difference between fractional freezing and fractional crystallization?
There is no difference. Strictly speaking, "freezing" does not necessarily include crystalline solids, and may include amorphous materials that precipitate out of solution. But, this does not really matter in the context of these eutectic (or similar) systems.

I suspect the latter is referred to as "partial freezing" because the solid is water, whereas it is more general in the former. We rarely speak of water crystallising to ice, but rather water freezing to ice. This is more of a linguistic issue than scientific.