I think you will find that you dollar has been bumped or bashed. it may have panicked and collided with some deccor or it may have been, involved in a bust up in the tank. however it there is a fair bit of white fluff, my guess would be that it is the latter.
i would advise to treat your tank with some sort of anti bacterial, as a precaution. but apart from that, there is not much you can do but wait. though perhaps keeping an eye on the tank to try and spot the culprit!
they heal slow, but well. in a clean environment. first the fluff will drop off, leaving the wound visible, over a few weeks the wound will slowly heal. leaving not much more than a slight mark.
All the best with this, and let us know how you go!