Black algae? Does it exsist?

[Acronuts]

Has anyone ever had a black form of algae grow in their tank?
I am wondering if this is something I need to worry about.

I also have been noticing that since I have been using a ca reactor my coralline algae has gone from pink and purples to more dark purples and even maroon red.
Could it be possible that my media in the reactor is too old?
Or am I just now seeing my tank mature?

js