I had the same problem a few months ago. I think it was due to both lack of flow and overfeeding. It does kind of clump up on the sand so it's possible to pick out most of it by hand. My outbreak was getting out of control so I used the extreme measure of adding Chemi Clean to the tank. It killed it all of within 3 days. You may want to turn off you skimmer because it will make it go nuts. After the third day, I did about a 20% water change and turned the skimmer back on. It took another couple of days for the skimmer to settle down but everything cleared up nicely.