Red Slime Algae

This stuff is nasty! Strange thing is Red slime algae isn't an algae at all. It is a colony of cyanobacteria that actually imitates a blanket of algae. It can come in a variety of colors from red to blue-green. (Most commonly seen is a dark maroonish color)
Like algae this bacteria needs light to survive and will cover live rock, coral, and anything else not move in your tank to get it.

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Solutions​​​​​​​​​​
Once it gets foothold in your tank, red slime algae is hard to keep from spreading. You will need to siphon away the bacteria during your water changes. However, this usually will not prevent it from reappearing. Also test your water for nitrates and phosphates. Consider feeding less, filtering more, or both.
Ultralife Red Slime Remover, Boyd Chemi-Clean, and Blue Life Red Slime Control all work really well when used as directed.

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Sources
http://kb.marinedepot.com/article.aspx?id=10984