Applying the wet drywall mud to the clean may loosen the popcorn texture due to adding moisture to the product. 

I've listed below the order I plan to attack things in, let me know if this sequence makes sense:Resolve excess moisture under house (minor grading issue, gutter required, possible plumbing leak)Repair minor roof leaks (repair flashing, seal leaks around chimney, replace tree-damaged shingles)Trim trees away from houseInstall 50 concrete piers and level foundation (foundation had work done previously but shifter due to excess moisture)Install vapor barrierAdd structural roof support beams in attic, add insulationRepair rotten siding/soffit/window frames (mostly upstairs)Repair porch (replace rotten boards, sand & paint)Paint exteriorInstall central AC/Mini-split system (any recommendations on this?)

Water is getting in somehow and has to be stopped or moisture will just come back again.As a landlord you are not a mold expert.

I would definately do a home and moisture termite inspection.  

Moisture meter only showed right in that immediate area.

I have a single unit with similar problems and it has problems with the floors being very cold, moisture build up, and of course mold. 

Any advice is greatly appreciated.8bd/4bth legal 3 unit Unit 1:2/1 TNT pays all, rent 550Unit2:2/1 TNT pays all rent 475Unit 3:2/1 rent 450Asking price 100kARV 140kDp 5% at 5k loan AMT 95kLoan interest at 4%Expenses:P&I: 453Insurance: 100 Water sewer: 100Prop tax:300Repairs: 147.50 at 10%Capex:147.50 at 10%Vacancy:118.00 at 8%Cashflow: 158.46Cap rate:5.25%Coc ROI:38.03%GRM:5.65This represents a purchase at asking price however when I viewed the unit a couple things stood out the attic seemed to have done a little moisture issues so I'm guessing the roof might need replacing, also the tenant in one unit had been therefore 10 years, thus the rental are below market value from comparables a these units rent could conservatively be rented for 600 600 and 525 boosting cf to 383.46, the first unit has lower ceiling also the one tenant who'd been there for 10 years complained of the first unit being occupied by gang bangers who sell drugs out the unit.

If the assumption is rust, then you can only conclude that it is the combination of metal and water(moisture).

Usually it will be pine or fir, sometimes poplar, at least with this sample you will know and can research if it is a bleeding type.You could also grab a moisture meter and do some tests in the middle of the run and next to the door, this will give you a little more information before making a decision on which way to go.I would also be looking for exterior leaks, has the caulking failed?

I believe a big cost associated with his work was due to moisture problems from the indoor pool.