I have a vinyl lined pool and the picture included is of a spot on the liner that is growing.
This is happening on side wall, this area of the wall was clean with no connections or otherwise.
It appeared last year and has grown (picture is from May 2015, almost a full year between them)
Any ideas on the cause and treatments would be most welcome.
Core pool info: 38K gal, vinyl, Intelliflo VS+SVRS pump, DE filter, liner replaced 2012