Welcome to TFP.
It is directly under a return. What type of filter do you have?
Have you tried to brush it? How long has this been there?
Need to provide more background information on what investigation you have done.
It has pretty much been there since the pool was installed, but seems to have grown in size. I have brushed it avidly and that doesn’t help. It is under 1 return. I have a Hayward cartridge filter.
I would recommend this as well Wet Edge has a 10 yr guarantee so file your claim and see what they say.
It appears to be blotching but they can do a better investigation.
Welcome to TFP! For a wealth of pool care information please check out our free Pool School. If you are in need of specific help then we encourage you to Register and ask us here in the forum!