Renzo,
A picture will surely help, but in educated speculation, two things come to mind :
1) Its due to some sort of scrape, possibly a cricket chew, or even a spider bite. A topical antibiotic is in order for such; or
2) Its fungal, due to too much moisture. Usually a brown scab/slough. If that's that case, go get something for athlete's foot, such as a Tinactin spray or cream, and give it a whirl. Spray or apply right to the affected area, and then some.
If its the latter, you need to look at how you make things wet, and ventilation. Things are staying too wet for too long. Sometimes a fan is all that is needed. If it is the former, especially from crickets, a larger cage and less free crickets are in order. It its spider, usually a widow, find it and kill it. Spider bites will take longer to scab over ... usually a week or two. Apply antibiotic daily until you get a hard scab.
Hope to see that photo ... take and post more than one ... thanks.