So did you also clean it off your rocks? And do I need to UV light? And did you not have your lights on your lid on during this time?Looks a whole lot like what I saw when I cycled my biocube. Even in the same spots.
I vacuumed it up and flipped on an inexpensive Amazon 9w UV light submerged in my pump sump (chamber 3). It was gone in a week or so. Haven't seen the same stuff on the sand since.
I discontinued using the UV light after 2 weeks. Don't know if the UV was really beneficial or not to help clear it up quick, could be it was a dumb thing to do, but the UV didn't seem to hurt anything either (other than giving my pump a slightly weathered sunbaked look on the plastic housing). If I ever decide to use UV again at some later date, I will likely go with a flowthrough design.