I found a bunch of what I think was black mold yesterday around my skimmer frame. I took a tooth brush to it and got a black cloud in the water. After the cloud dissipated I saw a lot of black particles floating around. On the plastic frame of the skimmer was a greasy black film which really didn't want to come off. I slammed the pool with chlorine. When I got home from work this afternoon I tested my chlorine to see where I was...it was all gone. I then tested my CYA, it too was reading zero. i went to the pool store and they told me I do have CYA, that my 'slam' has made reading it, temporarily difficult.
I decided to take a look at the filter cartridge, it was covered in mustard algea like I had never seen before. I cleaned all of the plumbing I have access to and replaced the cartridge with a spare. I also brushed the pool. While I was doing that my wife was spraying the cruddy filter down. She observed black mold in some of the filter pleats. I've heard about 'acid washing' the cartridge, is this the best way to really clean a cartridge that has mold or is there a perhaps less caustic method? Right now the filter is soaking in a cascade bath.
After all of this the pool seems to be holding chlorine again, thankfully.