a simple answer would be to slap on some "Nurgle's Rot" technical paint...however, that will give you a very strong bright green "cartoon toxic sludge" look - and if that's what you want, cool!
...personally though, I find that to be a little too intense for my stuff...I'll just give you a full rundown of how I do my slimy nurgle bases...
first off, slather on some dark brown texture paint, like
GW's "Stirland Mud/Battlemire" (I make my own, using ~half a jar of Atelier brand "Moulding Paste", topped up with sand, some fine cork crumbs - scrap from making cork rocks for bases - and some burnt umber craft/artists acrylic and a bit of water)...then let that dry, probably overnight.
when dry, drybrush with some Averland Sunset or Balor Brown.
then a all-over heavy wash of Athonian Camoshade
now for the slime...I do any slime with 'Ardcoat, thinned out with a bit of water so it flows better into pools, mixed with just a touch of Elysian Green, and/or maybe some Screaming Skull, or maybe some Ogryn Camo, depending on whether I want more "green", or more "pus"...I usually wind up adding a couple of slightly different coloured pools around the base, mixing them together at times too...don't be shy with it, apply it quite heavily, making nice pools of the stuff, because it will shrink down when it dries - feel free to add more on top later if it needs it.
when it's all dry, thin down some more 'Ardcoat, almost like a wash, and slather it all over the base to give the whole thing a nice wet, slimy look.
here's a few example pics of my nurgly bases: