Black areas on Acro

In just the past couple of days my green slimer has developed a dark gray/black patch around an area that was repairing nicely where a frag had been taken off.  It looks almost like a  fungus or something like that.   I did notice that my ph had climbed a bit in the past few days since I added a bit of washing soda to bring up the Alk.  Could it be related to a ph swing?

Any ideas??  I could probably post some photos of it if that would help in identifying if it is a problem or not.

Any help would be appreciated.

Thanks!