Odd discoloration of only white plastic parts below water level

Fuldo

Silver Supporter
Nov 23, 2017
227
Port Orange, FL
My daughter's pool in NH is a large three year old in-ground vinyl pool with white plastic stairs and swim-out. Today I was asked about this yellowish-green discoloration of those plastic items below the waterline. The water quality has been good (I've been monitoring/adjusting it), the water has been perfectly clear and the liner doesn't seem to have the same problem as the white plastic parts. Even the white plastic trim around the skimmer opening shows this same problem. The discoloration is slight but obvious and the surfaces don't seem slimy. The other odd thing is there are a couple of rectangular areas that originally had labels on them and those areas are still perfectly white. It seems the discoloration can be scrubbed away but a regular pool brush won't do it.

I suggested they remove one of the skimmer trims and soak it in a strong bleach solution to see if that removes the discoloration but that hasn't been done yet. I'm thinking it's likely algae by the color but it seems odd it would only affect the white plastic items.

Is there anything but algae that can cause this type of problem?