Plaster

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 could be blotching. Did you discuss this with the plaster installer in the days following the installation?
 
Have you followed up with your PB to let them know it hasn't gone away?

Send the pictures to Wet Edge too.
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.