I have seen this type of patch before. Its hard to say exactly how it happened, does the heater have a guard on it? It looks localized so I wouldn't be too concerned at this present time, give it 2 or 3 days, if it appears to be getting worse then add 1/2 oz per gallon of aquarium salt to help it heal. Like I said, observe it for a couple of days first, check the parameters are all good, its certainly not immediately life threatening, my guess is it will improve without the need to treat.
Good luck.