It is easily understood. I perceive the writer as trying to be colorful, originally, witty, and droll. In my opinion the writer succeeds. Of course, it all depends on the context.
"Sluice" is within the ordinary vocabulary of someone with ordinary education. It is used in the US. The first thing that comes to mind when I heard that word is the device used by California gold miners during the 1848-9 gold rush, to produce a rapid-moving flow of water, to separate gold from dirt. A YouTube video shows "how to use a sluice box." https://www.youtube.com/watch?v=gxe4N9fUhuU
The obvious idea in your passage is that there is not only slushy mud, but that it is actually flowing rapidly. The absence of any analog to sluice gates means that this is not a wonderfully precise analogy, but the author probably likes the alliteration of "sluice" and "slushy."
One form of wit is the intentional use of formal and elevated language in a situation where colloquial language might be expected.
If you've ever seen the movie, "The Wizard of Oz," an example of this kind of wit would be the scene in which the Wizard calls the Tin Man "You clinking, clanking, clattering collection of caliginous junk!" He is trying to sound dignified, is trying to be particularly witty by using alliteration, comes up with the excellent word "caliginous" (which I never bothered to look up until just now), then apparently runs out of ideas and falls down to earth--has to connect these words with the everyday and non-alliterative word "junk."