No idea what it is, but we've had great results cleaning the waterline & marks off our fiberglass pool using the Mr. Clean Magic eraser.

Sounds to me like it might be a stain from something metal that sat there for awhile like a rust mark. If the surface is smooth than it's just a stain and I would "try to" ignore it. I have found on my fiberglass pool that stains work themselves out over time.

Hi..have a fiberglass pool here. Does sound like a metal stain. There is an easy way to check. Take a vitamin C tablet (ascorbic acid) and hold it again the area for about 30-60 seconds. Lift off and see if the spots looks clean again. If it "disappears" its a metal stain. Ascorbic acid (vitamin C) will lift metal stains for fiberglass pools. I used a metal out product which bonds to metals in the water and helps filter them out. Anytime I add water I add a few ounces of Metal Out. Once I got lazy a few times and didnt add it and start with light brown dirty areas in the pool. Had to buy ascorbic acid powder and sprinkle into pool and filter 24/7. There is more to it than that but if you need help on a total metal cleaning I will be checking in here when I can. Hope that helps and let us know if that solves it.

Ls

New fiberglass pool last year and one day I saw what looked to be a worm or one of those 2 inch long tree pods laying on the bottom of the pool. I didn't go to the trouble to get it up at that time and a day later when I did, there was a rust spot just like you described when I went to vaccuum it up. I tried the magic eraser and scrubbed and scrubbed but to this day a little faint mark still remains. It is getting more and more faint as the months go on. I know this may not help you, but it taught me a lesson to get stuff up right away when I see it.

I had some trashy guests in my holiday home and they threw cigarettes butts in the pool. There are now brown nicotine? stains where they had sunk, also bobby pin rust like stains. It's a gel coated fibreglass pool. Does anyone have any ideas of what I could try to lift them. We had it re-coated approx. 5 years ago.