Hmm... that doesn't look good or normal. That's not a brown jelly-like material there is it? If so, Google "brown jelly disease" for more info. Highly infectious.
If that's what it is you'd want cut that head off, put it in a bag underwater to maker sure the junk doesn't spread, and get it out of your tank.
But if it's just a discoloration of the tissue, I'm not sure what's going on there.
