I have been having the same problem,,,although I have tried just about everything and I have been battlling this for a year,,,,mine is purple and almost black to dark greay,,,,it blankets the sand(velvet looking) and grows in long stings,,,I have tried not feeding as much,,,different lighting,,,more frequent water changes,,,nothing is helping,,,I just wait for it to grow and suck half of my sand out with a siphon trying to remove it from time to time,,,,I would also like to see someone else with this problem and pictures,,,and a Solution,,,thanks