Hi all ! 
First of all : I’m not English or American, so I don’t know if it’s the good word to say it 
But I would like to know how to create some mildew on concrete, I want to “put it” on another material that I’ve already created, for the mildew, I would like that it’d be looking like this :
(I want only to do the mildew, I don’t want the concrete on the image)
If anyone has some idea to do something who look like this I’ll be glad !!! 
There are a few ways to do this that I know of.
One method is to use texture paint on a new black and white texture where you want the algae/mildew, and then use this as a mask for the fac input of a mix shader.
The second method is procedural and much the same. You mix two shaders together using the fac input to control where the mould/algae goes, like below. The colour ramp allows finer tuning of how distinct the patches are.
Obviously you’d want to spend more time tweaking the size, colour, bump, etc.
Ok ! I’ll try that right away ! 
sorry it will take time before I test the two option, my computer is laaaaaaggggyyyyy
Thks for the solution you’ve give me, but I think this is not perfectly what I want after try it, but I’ve learn things with you’re post and I’m gratefull to you for that !
