You hate to see things like this when winter season comes around due to the fact that they're metabolism is slower but luckily your temps aren't as cold as some of us.
In the meantime until someone more knowledgeable chimes in, I'd just t keep an eye on it and hopefully it will heal on it's own.
If it doesn't go away you can do several things such as Iodine treatment, a salt or PP paste.
( warning PP can be dangerous to you)
Good luck,
Bonnie