Looks like you have some Cyanobacteria. There are multiple threads on here concerning it, its causes, its cure. Here is some info, use the search function here to find more.
Cyanobacteria, AKA Red Slime
Here's a little background to help you understand it.
Cyanobacteria in itself is pretty interesting stuff. Cyano is one of the oldest known fossils, more than 3.5 billion years old. This says a lot for its ability to adapt. And though we find it unsightly and it can be destructive to an extent in the marine tank, it is one of the largest and most important groups of bacteria. Among other things, it's an important provider of nitrogen fertilizer in the cultivation of rice and beans. It is photosynthetic and can manufacture its own food while producing oxygen. It also feeds off of organic materials in our closed systems. Not all of these organic substances can be removed by skimming and despite best plans and actions aquarists frequently run into some cyanobacteria problems, especially when their systems are going through initial cycling and settling in. Slime algae" can come in shades of blue-green, red, brown, and black.
What causes it?
As mentioned previously, it is caused by excess organics in a system. This can be brought on by excess feeding, overstocking, lack of filtration, infrequent water changing or from the excess nutrients from cycling a new system. Also, because it is photosynthetic, long light cycles encourage growth.
How do I get rid of it?
The best way to get to battle cyano would be diligent tank maintenance. A combination of regular water changes, great filtration, not over feeding or over stocking, manual removal and good water flow. Regular water changes will help remove organics and lower nitrate and phosphate levels which are large fueling sources. Regular testing will help you keep your level in check but, keep in mind that test kits can only measure inorganic phosphorus. If any phosphates show in your testing, this means there is a higher amount of organic phosphates. Anything above 0.03ppm should be lowered asap. I feel at this point I should also add that using tap water may be adding to the problem. Testing your tap water would be advised. Some tap water contains phosphates and nitrates in which case RO/DI water would be necessary for top offs and salt mixing. Other things that may help are adding more waterflow to your tank. Cyano will thrive in low flow stagnant areas. Manual removal will help as well. Siphoning the cyano from your sand/gravel and rocks with airline tubing will help remove bulk amounts of cyano. Be sure to remove any cyano that you wipe from the sides of the tank as well. Feed less. Just because the fish swim to the top like spoiled dogs begging doesn't mean they need fed every time you walk past the tank. Actually, it's good practice to skip a day of feeding. Many aquarists feed every other day. Commercially prepared foods contain a lot of phosphates as well. A homemade frozen blender mush* is a good alternative frozen food. This is a mix of fresh seafood and vitamins thats not full of preservatives. If feeding daily, feed small amounts so there isn't a lot of excess food laying around. If feeding frozen foods, thaw the food in some tank water and drain before feeding. Care should also be taken when feeding inverts/corals. It's easy to overfeed so be wary of adding too much. Few animals will feed on cyano and only as a last resort for food. Some hermits may pick at it and nerite snails are said to eat it but the only true consumer is strombus sp. Introducing macros that will compete for the nutrients will help as well. Lastly, there are several chemical products on the market that can be used to control cyano. These can be a mixed blessing. While it may control and/or eliminate your cyanobacteria, these fixes are usually temporary. Cyano being so adaptable, unless you change the conditions in your tank that is causing the problem in the first place, it will often adjust itself to the new chemicals and reappear over time.
