Stain on Fiberglass steps

I had waterline stains on my fiberglass steps, they were sort of grayish greenish colored. Wierd thing is, we also have a fiberglass swimout, and there were no stains on the swimout. Anyway, I had read that someone tried the Magic Eraser, so I tried it and it worked like a charm!!! My question is, should I further investigate the cause of the stain (metals, etc?) or just be thankful that all seems OK? My numbers are:

FC and TC 5.0
CYA 50
PH 7.6
TA 140

I use bleach to chlorinate and shock, and trichlor sticks when out of town. Pool was filled May of this year, and I've had nothing but clear blue water all season.

I would not worry about any other stains - it was probably some kind of build up, and the magic eraser worked. If you really had a problem with metals, there would be more stains. All of your numbers look good, but I am not sure how you measure your TA. If it is just alkalinity, then it is a little high - Alkalinity should be between 80 & 120. But if your water is clear, I wouldn't worry about it.