I have run a Mineral Springs salt water system for my pool which has a vinyl liner. For several years there has been a black slime that forms behind the underwater pool lights (which are rarely, if ever, turned on). I was told this was because we have an in-floor vaccuum system (Vanquish) and need to run the returns more frequently. This year, this black slime seems to have migrated to my pool inlets which have formed buckled ridges in the liner around the inlets. In addition, it is under my ladder rungs and upon removing the ladder appears to be IN the aluminum railings. I'm not 100% certain, but it may also be collecting at the seams. The water is otherwise clear and appears to have good chemistry but I have no idea where this coming from and how to prevent it.
The structure of the pool is a padded alumuminum frame with a vermiculite base. Could there be something between the vinyl and structure that is "seeping" into the pool?