Live Rock has big black spot??
 

One of my Live rocks has a portion the size of a toonie that has turned from purple to black??? What could this be and should I be concerned?
If so how to get rid of the black and not damage the nice rock?
Thanks all,
Steve'o

Sponge possibly? Have a look below.

http://www.reefcentral.com/forums/sh....php?t=1140423

Post a pic if you can, will make it much easier.