Contractor Repair Marks
Ah the simple X marks the spot what else can you
ask for when inspecting a home. This is a very good clue and could have
been made by homeowner or contractor as a sign it needed to be replaced
Experienced home inspectors will be looking for clues like this during their home inspection which in turn will help with their investigation of the property.
Looking at this tread when I took the picture, it seem like the only damage was what you are looking at and that seemed to be black mold.
Just because you see an X don't assume there is a problem, just give this stair tread a little closer attention. The mark could have been made during construction in the paint was unable to cover it up or the mark bled through after time.