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IIt looks like either Diatoms or Cyanobacteria IMO.
http://www.reefcleaners.org/nuisance-algae-id-guide
Looks like diatoms based on the picture... What color is it under white light?? and does it look like slime ?
BrownIs it green or brown?
It is cyanobacteria maybe combined with diatoms. The cyanobacteria can move gliding and they move into the ground to refresh their intracellular nutrient stores. Especially phosphate concentration is much higher in the ground and the cyanos store phosphate as polyphosphate. In the morning they move back to the light for photosynthesis.
Yes, it is best to remove them with a gravel cleaner repeatedly. Vacuum them out with every water change.