You have what is called calcium cylicate hydration 'The formation of glass', I would bet that you have low calcium fill water and the ponding water never received calcium chloride at startup as reccomended by the national plaster council in there technical section. Since they plaster using calcium to speed up the set rate the higher calcium concentrated areas are the dark areas in your pool. The plaster has a inability to hydrate while the plaster is maturating calcium to the ponding low calcuim tap water. The difference between the tap ch ppm and approx 220ppm was extracted from the plaster. If you did add calcium at what rate did you add it and what purity level %77 or %94? And no that's not under any warranty since warranties only extend to material and workmanship. If you have a low carbon alkalinity this would be called the 'deadly Duo'. If you have severe dark spots they may need a carbon heat treatment. for light discoloration throughout a 0 alk procedure will hydrate the effected areas in 7 to 14 days depending on water temp. Ie. moisture is trapped in the plaster. Austinproleak.com Here is a link that might be useful: austin pro leak

It sure looks like mould to me. If it doesn't smell, perhaps it's not 'active' right now, ie it's dry. Is that an outside wall? Definitely pull off the drywall, you may get more clues then. When you resolve the issue, I'd use a mould-resistant (humitek or similar) drywall, but that's not a solution - you need to get to the root of the problem. As landlords you could be in some trouble.

Probably this pool at its age has been "acid washed" several times...what these people will not tell you is that each time you acid wash you etch layers off of the interior plaster. This cannot be repaired unfortunately. Your best bet is to have the pool professionally replastered with a premium product such as PebbleTec otherwise you'll have the same issues with the low quality plasters on the market today. The existing interior plaster of the pool will have to be chipped out, any repairs made and then replastered.

It's either the heat of the fireplace or the combustion products from the fireplace affecting the tile. Got a spare tile? Put it in the oven and age it at an elevated temperature. If it discolors, you know its the tile. If it doesn't, it's the combustion products. A ventless gas fireplace is a very bad idea! Besides the carbon monoxide risk, putting all the CO2 and water vapor from combustion into your home is not a smart thing to do. Definitely go with a direct vent unit. https://basc.pnnl.gov/resource-guides/unvented-combustion-appliances