Off the top, I guess you could go about it in two ways...
You could load an image of a drop of blood, make it appear when you click and then lower it's location each frame.
You could make the blood via a curve or bezier, make it appear when you click and then lower it's location each frame.

Here is the reference for mouseClicked() and loadImage()

Yes, i thought of that, but the thing is that i want the line of blood dropping down the screen to be irregular, like a real drop sliding on a wall...

I'd do it like this:

• don't clean the screen between frames (that way the blood will "stick") or use fill(0, 1) and rect(0, 0, width, height) for a slow fade-out
• make a particle that slowly falls down whenever you click a mouse, and which draws a red spot on its current position every frame