under that is a soggy brown slutch which I presume is the stuff
that hasn't had enough oxygen to decompose properly. If I leave it
open to the elements, having taken off the top layers, will it
decompose by itself? Should I add anything to it to start it off?

The well rotted stuff is a bit fibrous still. I am therefore mixing
it with mole hills
snip
Isn't the slushy brown stuff just all the goodies leaching out at the
bottom and therefore well rained on, rotted, washed through, etc? I
don't know but I think it's a fair guess.
Sounds as if you've got a little brown mine there. ;-)
--
Sacha

Yes indeed that will be great stuff, if it is too slushy to use get some
straw and mix it all up a bit or I suppose that you could mix it with
water and use it as a liquid feed, have fun :-)

kate