Quicksand of prejudice means that prejudice cannot form the base for anything stable to stand on it.
Explanation:
Quicksand is a colloidal substance which consists of granular molecules and water which is mixed into it. When the water inside the material cannot escape it makes it mushy and loses its agility.
IT loses the ability to hold the things that are put into it and is wont to crumble.
So in terms of the prejudice that is connected with quicksand it is evident that the person is trying to convey that like the quicksand here prejudice cannot be the basis of any stable structure because it will crumble down.
