Actually, routing wash water into a drywall is much better for a septic system than into the tank; high loading is the quickest course to failure for a septic tank, not to mention bleaches, heavy phosphate concentration, etc wreaking havoc on the bacteria population.

At the very least a power wash and bleach to clean them up.  

Check for flooring surface quality as I see some bleach effect in 2 of the living room photos suggest they got more under the carpet as well.  

I'm going for some 400 thread count, middle-of-the-road sheets in stark white so they can be bleached. 

We cleaned it up (wearing gloves & face masks)....lots of bleach. 

Sung to the tune of "Killing Me Softly"Pushing the turds with my broomstick,Spilling the bleach on my pants,Throwing my shoes in the garbage,But they're paying my mortgage, my mortgage,Paying the whole thing, my mortgage, 'till it's gone...It's been a short 12 hours,Since I sat at the bar,Explaining to my friends howMy rentals will take me far,A distant memory as theClock rounds 12 and I'm still singing,Pushing the turds with my broomstick,Spilling the bleach on my pants,Throwing my shoes in the garbage,But they're paying my mortgage, my mortgage,Paying the whole thing, my mortgage, 'till it's gone...It's true, the first of each month,I'm cashing checks at the bank,It's true, from time to time thatThe money's gonna stank.I'd say this night is reallyPutting the "a**" in passive income,Pushing the turds with my broomstick,Spilling the bleach on my pants,Throwing my shoes in the garbage,But they're paying my mortgage, my mortgage,Paying the whole thing, my mortgage, 'till it's gone...

Have the basement fixed, clean the walls with a bleach spray, put in a dehumidifier and open heat/cooling vents to air out the basement. if that doesn't completely stop it, drylok the walls.

Wore my worst clothes , had bleach water and soap and had a miserable day .  

It's not uncommon for tenants to try and remove a stain using bleach.

Just get rid of it and replace it something more durable that can be cleaned with a mop and some bleach.