Hi – yes, you can use stain free on the stain. It’s a powder based ascorbic acid and you would just dump it out at the wall right there to hit the light area. the Screw holding the light in place could be corroding and it could be as simple as changing the screw. If the housing itself is causing the rust then figuring out what part is doing it would help. Alternatively you can rub actual vitamin c tablets on the stain and it will come off in seconds. The stainfree does cause the chlorine in the pool to be used up, so once the stain is gone – let the filter run for a few hours and then shock the pool with chlorine and you should be fine.
Also – a magic eraser should also be safe on the ecofinish – but it’s hit or miss whether that removes staining of rust – sometimes it works sometimes it does not.

Make sure to double check your pH and alkalinity levels – aggressive water when those are low contribute to rusting, even of stainless steel in extreme cases.

Best of luck!