We had similar problem that I thought was algae but after shocking and maintaining at high levels still had this stuff. (Water was too cold to get in to check if it was slimy or gritty so I assumed the worst.) It was gritty when I could finally stand to get in to feel it.
Next thing that was suggested in the forum post was the spider gasket on the sand filter was bad...sure enough it was. We had performed no maintenance on the filter in the 9 years we have had pool. We ended up replacing sand since we had everything torn apart anyway and wanted to check the laterals too.
Debris is much much less. We still have some because I have a dog that runs across the top step to drink and wash her feet and belly while she is at it. AND it has been bone dry with dust, dust and more dust!!!