Brown on sand appears with light, disappears at night

So I have my tank very dialed in and its darn near perfect. I never thought I would get it to this level. I only clean the glass once a week and even then there is near zero on it.

Let me give tanks stats.
red sea and hanna tests

Ammonia: 0
Nitrite: 0
Nitrate: 0
Phosphate: 0
Temp: 79
pH: unknown
ca: unkmown
alk: 9
salinity: 1.025

40 gallon tank. I run GFO and bio pellets. I supplement microbactr7 2.5ml once a week. I feed daily red sea reef energy part a and b 2ml each. I have LED lights on 12 hours a day but peak light only 7 hours. The rest goes through sunrise sunset. I do a 20% water change once a week and only use RO/DI with TDS tested every use and never use water if anything but zero. I siphon the sand every week.

So to the problem. I thought I had a classic diatom problem. Its the brownish dust covering the sand only. Tank is now 9 months old so no idea where silicates would come from

When lights are off, sand is perfect and clean. Once lights turn on, over a few hours my sand turns brown and looks identical to diatoms. Once lights off again for about 2 hours, sand is perfect clean again. Corals are unaffected.

What can this be? I do not have a photo and I'll try to get one tomorrow