From a site about making home brew:
The brown scum that forms during fermentation and clings to the side of the
fermentor is intensely bitter and if it is stirred back into the beer it will cause
very astringent tastes. The scum should be removed from the beer, either by
letting it cling undisturbed to the sides of an oversize fermentor, or by
skimming it off the krausen, or blowing off the krausen itself from a 5 gallon
carboy. I have never had any problems by simply letting it cling to the sides
of the fermentor.