rust color spots

[tomr]

First time in 25 years have problem with spots on liner.
Pool supply store asked if I have a pool heater,yes installed 1 year ago
(16x32 pool inground) He said it probably is a copper stain. I tried
cemicals he supplied to no avail.I tried black algae cleaners too.
What happens is that in the early morning the pool is CLEAR, no stains.
But as the day goes on the stains come back.
Does anyone have an answer to this?