From www.swimmingpoolsteve.com/page... - Learn what can cause your swimming pool plaster to have brown stains. Pool plaster is essentially white sand and white cement which is porous and can stain easily if left unmaintained. This is especially true for new pool plaster as new pool plaster can become stained very easily. Great care must be taken to protect new pool plaster from stains. Iron can also cause brown staining and could be coming from broken or old pool equipment. When brown stains suddenly appear on pool plaster swimming pool technicians will often inspect the equipment specifically for a failing pool heater.