cant see properly...is that like a water mark ?

it could be due to water entering your wall from outside....u need to check if there are any leakages or holes on the wall...i use to have this problem and took a while to finally figure out the problem....

do u have an ac compressor on the outer part of the wall? if yes, check and see if the aircond piping from outside, where they plaster the hole..check if the hole is fully plastered..if not..could be water sipping thru from there...

good luck