it's cyano bacteria, just not very colorful, lol.
Cyano bacteria is caused by many things, so it's typically a pretty good combo of problems. I typically think of two things.
1. Excess nutrients. This would be caused by overfeeding the tank, failing to do water changes, or having poor protein skimming.
2. Flow. The reason people get the cyano on their sand, as in your case, is that they do not sufficient flow in the tank. This means that when you feed the tank, the food falls to the bottom, breaks down, and creates cyano.
I would say you have both. Get more flow goin' in that tank, it'll help you IMMENSELY! Your livestock will be healthier and happier, and your sand/tank will look cleaner. Not to plug myself, but a couple K4's in that tank would be sweet, and I have a special on them 2 for $80!
But your biggest problem is the nutrients. Watch your feeding, and invest in a protein skimmer (the first time you empty the collection cup you'll wonder how you didn't have one!) and secondly, from looking at your rocks I am assuming you are using tap water, if you plan to have a great looking tank over the long term, invest in an RO/DI unit (going to your LFS is a pain). Tap water has all kinds of gunk which just creates unhealthy water and dirty algae filled tanks) One thing i've learned in this hobby is that NONE of us keep fish and corals, we keep water. Keeping water healty allows corals and fish to live. Easiest way to be successful is to start with great water.
