I took the cover off yesterday afternoon. There were a few leaves that had got in as I was closing last year. I got them all out with the net. The bottom looked great! Then I turned on the hose (municipal water) and started to bring the water level back up to mid skimmer. I ran the water for about 4 hours then shut it off for the night. I turned it back on this morning for a few more hours. The bottom was still clear. I shut the water off because I had to leave. Mid afternoon when I returned I turned on the water again. It got to where I could turn on the pump and start circulating. I'm not sure if it started before I turned on the pump or after but I noticed this brownish rusty colored stuff on the bottom of the pool after uncovered only 24 hours. It wasn't above 62 degrees here and hasn't been so I don't see how it could be algae. I fought with whatever this is the end of last season. But it wasn't on the bottom when I opened it. I just finished filling it tonight and plan to start with chlorine and such in the morning. What could it be?