My first dog had a similar lump on her hind leg the size of an egg. We had a needle biopsy done and they said it was benign not to worry. It continued to grow to the size of a baseball. We had it removed because even though it did not bother her, it did not look nice. She never had a problem after that with her leg. I suggest taking your dog to a Vet just to be sure it is not cancerous.
It may be just a wart (some breeds can get a lot of them and sometimes they kind of bust open) or skin tag. Are you sure it's not a bite from a spider or something? I probably wouldn't worry too much unless you start to see signs that something is wrong in your dogs basic behaviors, eating, drinking, or the leg starts to swell or it grows or you see puss or anything else 'off'. Otherwise you can probably wait to ask your groomer or vet about it next time your there and keep an eye on it and your pup for any changes.