Crushed Coral Discoloration
 

Certain sections of the crushed coral in my tank have just recently become discolored, it is a murky brown color, what are some possibilities for this? Is there any way to clean it?

Diatoms most likley. How old is the tank? If it less than 6 months it could easily be diatoms and even if it is older it is a possibility.

the tank is about 3 weeks old, what are diatoms exactly?

Diatoms are a microscopic something that feed off of silicates in the tank. Once the tank is old enough the diatoms will be gone naturally as the available silicates are covered with the natrual bacteria and the diatoms can no longer get to them and eat them.