This is normal. The sludge is oxidised hop proteins that often appear as a dark brown/black sediment on the sides of the fermenter at or just above the wort level.

To clean your fermenter;

1. Fill the fermenter with a warm cleaner solution Grainfather High Performance Cleaner, PBW or another similar cleaner
2. Allow to sit for 5-10 minutes based on how bad the sludge is. 
3. After letting the solution sit, slowly allow the cleaner to drain while using a dish brush to clean the sides.  This will remove all the proteins from the fermenter so that it is ready to be used again.

Before	After