Is it a stain, or my lying eyes?

[mbar]

No, your eyes are not playing tricks on you - it probably is a mineral stain. You can take some vitamin C tablets and crush them up in a sock, or ph down, and rub it on the stain - if it takes it up it is a mineral stain. You can take your ph down to 7 - 7.2 and put in some metal remover (sequestering agent) and it should lift the stain. Let us know if this takes care of the problem. I have a fiberglass pool, and if it has stains you mostly see them in the pool when there is no sun on them.

[steveinaz]

What makes testing the stain difficult, is if the lighting changes, the stain disappears. I'll give it a shot though.

[mbar]

If it is that hard to spot, then I would ignore it