stain not removed by trichlor

[rnjw]

I tried rubbing a trichlor puck on several stains on the bottom of our inground fiberglass, painted pool. Although one stain appeared to show a clear line through the rubbed area, another appeared yellow after I left the puck on it for about an hour.
Have not gotten any citric acid yet.

Hard to get a photo that shows clearly as the stains are mostly low contrast against the bright blue.

Welcome your suggestions.

 

[Butterfly]

You can try cheap Vitamin C tablets for the ascorbic acid test.