Some horses suffer from this but it does not bother them unless you distrube it, making it soar etc.
If she's not headshaking or in any obvious discomfort and the problem is not getting any worse, then you might be best leaving it be but your vet hopefully will be able to tell you exactly what it is.
Rubbing oil into the area will soften the scabs which you carefully rub over and remove with cotton wool, then you will be able to treat it with a cream etc.
Thornit powerder is also very good for ear problems in both dogs and horses, this can be blown in by a dispencer.
It could be ear mites?? You could ask your vet for some drops. The other possibility it could be caused by the herpes virus ( don't worry and its is not infectious). Our old horse who is 19 now had them when we bought him when he was 12. Never caused a problem. I would clean the ears with a baby wipe and carefully apply some Vaseline ( only apply a thin layer) to soften scabs and get some ear mite drops from the vet. ( Better safe than sorry as ear mites can eventually cause problems if left untreated and can be very uncomfortable for the horse.