Step-by-step:

Creating viscous-looking nose slime

1 I create a new layer and use Photoshop’s Pen tool to draw a selection path that will become the outline of the slime trail. I fill this selection with a slimy green colour, which I use as my diffuse colour base. Then I lock the transparency of the layer. 2 I brush in dark values where there’s less light, sampling from the background and painting in some of that light colour. I’m suggesting translucency by lightly painting in what’s behind it. I avoid using an airbrush because I like having painterly edges in my art. 3 To reflect the main light source, which is from the top left, I add crisp, bright point highlights. Then to match my line-art style I outline the shape of the slime trail. To further suggest volume I add rim light reflections, from the secondary light source to the right.