I see the photos, but they aren't sufficiently high resolution for me to have any clear idea of what you might have.

You are in Hawaii, so getting a K2006 may be hard. Can you get this kit from Walmart? http://pool9.net/hth-6way/ (Unlike you, I do NOT trust your pool guy's reading.)

It's going to be hard to put Vitamin C or Iron Out on those spots, since they all seem to be on the wall. Can you get a sink plunger like this one on Amazon
to stick to the wall? You'd probably need a wooden handle one, so you could unscrew the handle.

If you can, put a handful of Vitamin C tabs in the cup, and then stick it (or hold it) to the wall for 15 minutes or so. See if there's any change. Do it again with 1/2 cup of Iron Out. And do it a 3rd time with pH Down (sodium bisulfate), but don't leave it there longer than 5 minutes.

See if any of those produce a change.