It’s time for the slime!
In my initial sketch, I focus on creating the flow of the creature and the dynamics of how the material would move. I include bits of slime that run and droop, bits that fling through the air as a tentacle flips or an arm moves, and parts that clump and smear as they drag on the ground. I keep the pieces ropey so they feel less watery.
Slime is shiny and irregular. To help create this look I use the Lasso tool on an Overlay layer to paint in the harsh and bumpy shapes of the shine, then softening them with an Eraser as they turn across the form. A Scattered custom brush can come in handy around these harder core highlights, to help round out the form as well.
To create a creepy background to ground my creature, I use the Quick Mask tool (C) to block in the areas I want to work on. Inverting the selection and making a copy via Copy Merged, I create a layer that only has the background area. Now I can paint behind my figure with as many layers as I want, simply by making them clipping masks.