Why do I have diatoms in my sump?

hey guys... so my tank is 2 years old, as most of you know I installed my sump not too long ago.. but I have diatoms growing in my sump tank, not the DT. Not of epic proportions.. just some brown on the glass and I wanna know why and what I can do about it. I know for a fact they are diatoms because I looked with my compound microscope and then brought them to my lab and looked at them with a confocal microscope and a scanning electron microscope. I even identified the genus and I'm pretty certain of the species. Anyway...

My RO/DI is putting out 0 TDS, so no silicates there.
My nutrients are all 0. literally everything.
Calcium: 450
Mg: 1350
Alk: 9 dKh

I have zero nuisance algae growth anywhere except these diatoms, which obv arent algae. But why are they here?

I run phosguard which removes silicates as well so I'm confused. I know the sand bed has silicates in it, are they leaching out?

I just wanna get rid of them because tbh, my water chemistry is so unreal good right now that if I can get rid of them, things would be perfect.

Like I said, its not even a huge deal, just annoying.

I'd like to hear your thoughts. Thanks.