Agree, to me too it looks like the beginning of an Interdigital Cyst. I believe there is information on this site about them if you do a search. You can soak the paw in an Epsom Salt bath or can use a topical treatment. When Monty used to get them, I would apply Vetericyn wound and skin care and after a few days, all was well again. It is something good to always keep on hand too. They do need to be treated as they can get infected and get full of pus which happened once with Monty. Check underneath his paw between the pads and see if it is very red and swollen.