Is the phrase "Icy slush" redundant?

Wet rain or wet puddles would be redundant terms, since what is rain or a puddle but wet?

Slush, on the other hand, isn't as absolute a term. You could have some very heavy snow (wet snow), which isn't at all redundant, and is one type of slush. Icy slush would be a different type, being more ice than water.

More to the point, whenever I refer to "icy slush," I'm actually referring to the likelihood of ice being underneath it, as in:

The roads are very slippery due to a layer of icy slush. Drive carefully.

In the same way that an orange that is more red than yellow could be called reddish-orange, it is perfectly acceptable to call slush that is more solid than liquid icy slush.