It appears to be the same exact thing that happen to me before I joined TFP. In my case it was the chlorine (I was using PowerPowder Plus) and copper in the plaster/water reacting and creating a dark purple/blue stain on the floor of the pool.
Eventually it would fade after a day or two.
Do you know if you have been using an algaecide? My mistake was using the start up and closing kits from the pool store which contained algaecide with copper.
Thank you @JamesW! Got the grandchildren in there this afternoon with a sock full of vitamin C and they cleaned it right up. The HTH shock said it prevents algae, which I assumed was from the cal hypo as they only list cal hypo in the ingredients. So there must be an algaecide in the “other ingredients”.
Actually only reason I had it was that back in Feb/March. I started worrying about where I was going to be able to get bleach due to the virus. So I bought some figuring it would be better than using pucks. Turned out I was right about the bleach but I was able to get along with liquid chlorine from Lowe's instead. Thought I would just try one bag to reduce amount of chlorine needed. Lesson learned!