Attack of the moving carpet
 

I came home from work last night to my reef tank to find the whole sand covered in red slime algae. What is is this caused from and what can I use to control it? I used to think it was from overstalking, however it is mostly corals in there so that doesn't make sense.

Brutal stuff!

Not really an overstocking issue per se, but commonly suggested as such because the more mouths to feed means more food/nutrients which then leads to cyanobacteria. Generally speaking, a high incidence of nitrates is behind a cyanobacteria bloom although there can be other causes such as old lights (when I used normal output actinics I could tell nearly to the day when they hit six months old because the cyano would start, and disappear when I replaced them).

You can treat the tank with "Red Slime Remover" (or "Chemiclean" or other commercial cyanobacteria treatment) - it will turn your tank yellow for a day or so, and turn off your skimmer if you have one because it will go nuts - and then you need to do water changes plus run carbon after a day to clean the water out of the dead cyano, but it works like a charm. However it's only a symptoms treatment - if the cause of the cyano isn't found and addressed it will return sooner or later.