I have a similar problem with brand new FNS Plus Vert. DE Pentair filter, 48 sqft. IG Vinyl pool, 12K, 1 hp motor (1 speed), NoCal.
The difference being that we're still working on start up due to cartridge filter failing 2 weeks ago. So, algae problem. The difference is that
on the second grid cleaning we discovered that only a top 1/3 band of the filter grids got coated. How would that work with water exposed
areas getting coated only? I read "Use & Care". Of course we always wait for water gush before shutting of air valve.
Also, there's a decent amount of DE powder on the pool floor even though we used a kitchen scale to measure out exactly the recommended 5 lbs.
I'm asking here because if I call service ( our dealer and installer) I won't see them till later next week and there go another bunch of expensive chems..
not to mention that the water is warm & inviting....