That looks like cyanobacteria. Reduce nutriment, do water change and syphon out as much as you can and put ferric oxide in your filtration or skimmer.

You can also try to reduce the lighting period until it receade.

What are your nitrates and phosphates? You probably have a lot of dissolved organic in the tank and syphoning the sand and doing water change will help.

Red Slime Remover can help but it is just a quick fix and if the reasons for the cyano are not taken car of, it will return stronger.

You probably have a lot of dissolved organic trapped in your sand and syphoning that out will help.

thanks for the help!!! i appreciate it!!

I've never been able to permanently get rid of the cyano that appears on my sand. (I have large fish, and they eat a LOT) However, I can control it. What I do is vacuum the sand bed every now and then, and immediately follow the vacuum by 24-36 hours of darkness. I find that really knocks it back and it will remain gone for quite a while.

The darkness doesn't seem to bother any coral or fish. Sometimes, I'll even feed the fish a bit during the dark day.