Looks like detritus to me.. you should be able to brush it off with a toothbrush if it bothers you (although it will likely come back). Cerith, nerite, and trochus snails do decent jobs at eating it.
Looks similar to diatoms, however if proliferates during light cycle then it's most likely dinoflagelates. Unfortunately that's what I had on my rocks and sand bed and looked exactly like yours. Good luck.
Does it get worse during the day and clear up when the lights are off? Are you seeing bubbles form near the top of it? If not then your fine, prob diatoms.
Regardless, check your water, Reduce light to 8 hours/ or less, use a brush to remove what you can while vacuuming what you brush off, get some snails to keep the rocks clean, only use RODI water (don’t even rinse a cup in two water without completely drying it), and monitor your water quality Nitrates, PO4, Ph, and Silicates (if they are high). You may also want to use a phosphate removal media, such as GFO or Phosguard. GFO+BRS Rox 0.8 Carbon is probably the best combo, so long as you change it when it’s mostly expended. If you use a proper RODI water system like the BRS 150gal/day water saver plus, it’s unlikely you will have silicate or diatom issues unless you bring it in from another tank.