Green water only in the morning, sometimes.

The title pretty much sums it up, maybe once every week or two right when the lights kick on my water looks hazy green for about an hour or so. The tank is around 9 or 10 months old, 40 breeder, no sump or fuge. 120W Led lighting, HOB skimmer, deep sandbed. I broadcast feed the tank 3 times a week, and sometimes treat the fish with extra, assuming theyve behaved ;).
The green water seems to come at random and again, only right in the morning. I cant tie it to anything, the only thing that ever changes on the tank is lights on or off. My po4 is undetectable and my nitrate sits happily between 5-10ppm.
It doesnt seem to effect the corals atall all sps and lps seem happy as a lark.....as happy as a coral can look.
Im not sure if i should be worried or not, id just like to get some input on what may be the cause?

Thanks,
Cody