Welcome to The Forum!! (I love seeing pool dudes come here to learn their job better)

I had an issue with one of my pools last year that sounds kind of similar - the pool was cloudy, white/ bluish, and the chems were ~ perfect, the stuff wouldn't go away. There had been some construction done in the lot adjacent to the pool over the winter. I added DE to the sand filters and the problem fully cleared itself in a couple days.

I don't know if the 'DE in a sand filter' trick works for zeo, but, given that the debris is in front of the returns, you should look into the cause being something too fine for the filter to catch. If DE is contraindicated for zeo, you might have to use a floc.

Have a good and prosperous season!