Pebble Tec Stain

I have some floor returns... called the enviro tech system... just basically returns water via the floor in additional the side way returns... one of them has a yellow stain on the pebble tec around this return...
What should I do to figure out what is causing the stain? More than likely I think it's from my heater, as the by pass was closed, and when I opened it up, and turned the heater on... a lot of rust went into the pool....
What can I do to get rid of the stain?

If you think it's rust, try a Mr. Clean Magic Eraser. If you think it may be metals, then try crushing up a few vitamin C tablets, put them in a sock, or some ph down in a sock, and rub them on the stain. This will take off any metal stains. I have found that rust doesn't respond as well to the vitamin C or ph down - but the eraser works wonders! Hope this helps, let us know:)