Just commented on your other post but this looks like cyanobacteria to me, plus some other algae that it's growing over. Nutrient levels are more than likely high, causing the stuff to flourish. It could also be down to poor flow. I added a Vortech MP10 to my C130 to help my fight with the stuff early on but I still get some every now and then.
Manual removal is possibly, get a turkey baster and when doing a water change, just suck it up. That'll improve things initially, but you'll need to locate the root cause or it'll just keep coming back. Phosphate tests can show up 0-0.03, as the algae is using this up quicker than you can test for it. What type of water are you using? Have you tested the TDS (total dissolved solids) of the water before mixing the salt?