hi not sure what is causing,the black is dead tissue,would take razor and cut out divot ,removing all black tissue,do an iodine based dip,hopefully will stop the spread
Looks like maybe something was eating it and the small wounds got infected? In any case, fish guy is right and you should try to remove as much dead tissue as possible
Clean utility razor blade or surgical scissors and straight cuts. Remove any and all dead tissue from the tank. Healthy fragged pieces can be reattached using loose rubber bands to rock rubble.