liner mold

[bill c]

I have stain that appear in the deep end of my pool when the chlorine level is at the lower end of the safe range. The pool has a vinyl line and the local company that installed the liner says it is called liner mold and to ellivate the chlorine level. When I raise the chlorine level it goes away. It will not brush off. I never had this until the liner was replaced. Is there such a thing as liner mold or does this liner have some kind of defect?