Very likely decals, yes. We have some info on our wiki. http://wiki.polycount.com/wiki/Decal http://wiki.polycount.com/wiki/MultiTexture#Modulation_Blending
Some more links that might help you http://wiki.polycount.com/wiki/TexturingTutorials#Painting_Stylistic_Textures http://wiki.polycount.com/wiki/EnvironmentSculpting http://artbypapercut.com/
Here's a walkthrough of one artist's process. http://wiki.polycount.com/wiki/Roguedevelopmentdiary More stuff here http://wiki.polycount.com/wiki/Character
As Joao said, check the Wiki link up top. Some links to get you started: http://wiki.polycount.com/wiki/Texture_Baking http://wiki.polycount.com/wiki/Polygon_Count
The scratches are probably best as decals, while the mold could be done with texture blending. Another good trick is using trim textures. http://wiki.polycount.com/wiki/Modular_environments http://wiki.polycount.com/wiki/MultiTexture#Modulation_Blending http://wiki.polycount.com/wiki/MultiTexture#Trim_Sheets