I close my pool in Sept and don't look at it until May. I don't think I did anything unusual the past 2 years.

I was thinking that the stain may be caused by whatever ate the stabilizer, low PH, low alkalinity or a combination of all of them. I even thought that the stain may be from a rusty pool wall but my pool is only 6 years old and seems like it's in good condition externally

I agree it just looks dirty! Please let me know how you make out just in case I don't get to do it this weekend.