.- If there's water below the slab that's wicking into the concrete, the area under your new flooring will become moist, without any water ever showing itself above the new flooring - a breeding ground for mold.- If there's water below the slab with a bit of head on it (the water level wants to be above the slab), it will begin to push up on the slab, and may cause it to lift and crack.

Unfinished basements get moist but with proper air flow, mold/mildew has never appeared.  

Go under the house and see if it's moist or wet.Do you have an exhaust fan to expel the moisture?

five or six feet of heavy moist ground and water.so Drylok is good as is hydraulic/water curing types of cement, but these are band aids.The expensive fix is french drains around the house.

Even plywood with hot/cold moist/dry cycles will warp in time.

You don't want to be venting warm, moist air into your attic.  

I would probably piggy back some on Peter's idea and cite the lack of basic building knowledge some tenants have,  with interesting examples like....pulling out a dryer to get a lost quarter but disconnecting the vent inside the wall to then vent moist air into the framing causing expensive (thousands) rot repair.putting a tarp on junk (literally) but slanting the tarp angle right into the siding in rainy season (rotting your new siding--but preserving their worn out tires).attachments to clutter, rusty junk, semi-operational cars and illogical desire to retain said junk and debris.I sort of also fall back on the divide between some of the personal management skills of landlords and tenants (realizing it may be why they are tenants, for example). 

Some people are, sadly, obsessed with how much pigment is in 1/2 of a mm of skin.  

The extent of it indicates it has been in a moist environment for quite awhile.  

If you seed it and keep it moist it will come in well, especially if the weather stays on the cooler side.