That gallon jug of Matte Medium from Scenic Express will sometimes have an inch of sludge at the bottom.
The last time I did scenery I used thinned white glue and the results were much more durable. But using the white glue darkens the scenic material when it dries.
That sludge is likely mostly acrylic resin that has settled out of solution, with maybe a little solids at the bottom.
How durable scenery is when using either glue depends a lot on how much you soak it, or how many applications of glue (multiple scenery layers) you use.
One of the reasons I got away from white glue was that it turns natural soil darker... Matte medium will do the same if you apply it multiple times, but the effect is not as dramatic as it is with white glue. At least in my experience.
Jeff