Signs posted read surface treated for discoloration.

Primarily, a sealed envelope of building, 2 years of roof, health & safety items like adequate heating, venting, no visible discoloration (mold), egress from each bedroom, termites, peeling lead paint, drainage issues, secured railings, and as miniscule as trip hazards if they are having a bad day.

I would suggest a dehumidifier in the basement and just clean the "discoloration" yourself.

To me, it looks filled with streaks and patches and discoloration - brush streaks maybe? 

As for the interiors, check for discolored drywall in the ceilings, water pressure (especially if they have galvanized supply lines for the water), and water spotting under the sinks.

New flooring, taking out discolored blue carpet for a luxury vinyl plank.

If the filter is discolored and not dirty they probably burn candles or is caused by some type of smoke.

Also, it discolors very easily over time.

There was some white discoloration on the basement beams but it was very minor.

The water leaking (Dirty brown water) onto the tenants property caused staining and discoloration to the bedding and mattress.