If you can pull it away from the stuff underneath it's probably ok, it's usually the ones that are obviously attached you need to worry about. Quite possibly just a 'fatty lump'. If in doubt, take a trip to the vet and get them to have a look.
Copper had one of these on his head but the vet said it looked like a cyst (was a brown/grey colour size of a pea). I thought it had changed colour so I asked them to remove it and test it. It was completely harmless, just a fatty lump. Most likely yours will be the same. Next time you are at the vet get them to look it over. No need to remove it unless you're really worried (cost £166!).