So you have two.
Yellow stains on concrete block on basement walls.
Rinse off the wall with fresh clean water.
Unless you take every step to keep the soil surrounding the walls as dry as possible those ugly stains will just keep coming back.
Therefore the first step to correct any type of basement wall stain problem is to provide adequate foundation drainage in order to minimize the water infiltration.
The salts are present in the mortar blocks or concrete structure and the moisture is usually attributable to some building defect.
As the moisture passes from your walls into your basement air it leaves any minerals in the water behind on the basement walls in the form of this white powdery substance.
Putting aside for a moment all the whining i ve already done above about mold rot bugs and wet basement anxiety efflorescence on a masonry foundation more likely to be a cosmetic issue or a moisture diagnostic.
Apply undiluted white vinegar to the entire mineral deposit you are attempting to remove and allow the stain to soak for approximately 20 minutes.
Change the water occasionally and cover.
Efflorescence is created as moisture passes through the porous concrete in of your basement walls and into your home.
This can diminish the appearance of your basement walls.
When emanating from mortar in brick or block buildings efflorescence will appear as narrow bands corresponding to mortar joints.
If you notice a white powdery film on your basement walls efflorescence is likely the culprit.
Efflorescence and water stains may explain spalling brick or concrete block foundation walls observed in a basement or crawl space.
A the yellowish stains leaching through the coats of drylok and primer on your walls are an indication that water in either liquid or vapor form still is entering your basement.
When groundwater consistently infiltrates the porous concrete walls through capillary action wall stains and related problems can occur.
Use a hose and sprayer outside or fill a bucket with warm water and wipe over the wall with a sponge or cloth.