This might be a skin irritation or an infection, especially if only one paw is affected.
Unfortunately, there's not much you can do at home, besides looking for possible causes. The cat would simply lick any topical lotion off. If possible, visit a vet to have them find the cause (maybe a bacterial or fungal infection) and offer the correct treatment. If a vet visit is not possible, it's ok to wait and observe how things develop. However, if the paw gets red and swollen or there's pus, you must go to the vet.
Cats lack a number of enzymes, which makes them prone to contact dermatites and poisoning with things like essential oils, lemons, onions and other surprising things that are safe for humans. Since the bare skin of the paw always comes in contact with surfaces, you should make sure that all residue of cleaners are rinsed away from the floor and surfaces with clean water. You shouldn't use essential oils or cleaners with essential oils to clean at all.
I noticed that your cat has long hairs between his toes. These hairs can trap some dirt or cat litter, which may increase the risk of infection. Please make sure that your cat has clean feet until this irritation is healed.
In theory this could have been caused by an allergy, but most allergies become first visible in the face as dark brown spots. If your cat has any dark spots in the face, change to porcelain, glass or steel bowls for both food and water.
Another possible cause is compulsive grooming, which may have been caused by stress. The rough tongue of the cat irritates the skin and makes it prone to infections, which in turn irritate the skin even more.
And one last note: the claws of your cat are peeking out of the fur. This might be due to the way you hold the paw, though. They should not be visible when the cat is relaxed. Make sure your cat has a scratching post he likes to use to keep his claws short. I noticed that my cat likes a very cheap felt rug more than her actual scratching post. If your cat scratches on trees and other plants, the plant juices could also irritate the skin.
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