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.